It's one of those things "if you have to ask, it's not for you". Having been intimately involved in HP modifications for these cars for nearly 10 years now I think it's safe to say I've a few valid thoughts to bring to the table.
First of all, the SC 3.8 engine doesn't have unlimited potential. There are always those who like to say "I wonder what it could do if...." Well, the "if's" have all pretty much been answered. There will always be instances of people making more power or going faster than the naive public thinks they should, and on the flip side there will always be stories of money spent for no good and disappointing results. It's the nature of the game, or business depending on how you look at it.
A recipe for performance is only a recipe for mediocrity because you can't put together a performance recipe and expect that everyone will get the same results. A performance recipe by it's very nature is a "shoot down the middle" approach that will with a high degree of probability result in satisfactory results. This in and of itself isn't a bad thing, it's just that human nature wants to focus on the best possible outcome rather than a more reasonable expectation which is what a recipe or "recommendation" is going to do.
That being said, to be asking "what should I do", you are clearly looking for a recipe (and subsequently mediocrity) whether you are willing to admit it or not, and that being said Damon has a good point. An S-port blower is a worthy component. It's not anything special or fancy but it does work. Similar porting procedures can be applied to any m90 blower with decent results or you can simply buy a new case MPx and not bother with any porting at all. Performance for your dollar - eh - that depends on what you are willing to accept.
The only thing about the S-port that stands out is that it does not sacrifice low end boost in exchange for high rpm airflow. Sounds like the best of both worlds, but in reality it's more of a case of decent from both worlds. Low end boost is great if you don't want to run a lot of total boost or overdrive. 0-10% boost is fine but any more than that on an S-port results in excessive and damaging low rpm boost. The rpms that the stock 3.8 is most efficient at also corresponds with the highest efficiency rpm of the S-port blower. This means that in the 2500-3500rpm range cylinder pressure is maximized and detonation danger is highest. Trying to run an S-port at 15-20% OD, or in other words upwards to 17-20psi boost and you have a recipe for detonation on pump gas. Keep the OD down and you have a very responsive and torquey SC. Since the adage holds true (you drive torque and you race horsepower), this is a worthy goal of any purely street driven SC. Just don't expect to win any competitions. But you can have fun, and it's not all about winning, right?
What the S-port or any similarly ported OE blower lacks, is the ability to intoxicate one with horsepower. Horsepower is not necessarily all that useful on the street, but it sure is thrilling to experience. Having driven an S-port pretty much to its limits (as well as plenty of other ported OE blowers) I can attest that all are somewhat lackluster once rpms exceed 4000rpm. No matter what you do to an OE M90 or how you overdrive it, the problem is always going to be that your power (torque really) band begins at 3000rpm or below and never gets any better after that. It can hang on pretty well for awhile if you have the right engine mods to support it, but it will never thrill you or put butterflies in your stomach after 4500rpm. The reason for this is not that it doesn't work above 4500rpm, but rather because you've already given it "your best shot" before 4500rpm. No matter how you pulley it, you can't make more power (torque) after 4500rpm than you made before 4500. It's just all downhill after that.
The MPX alters that curve somewhat. By matching low rpm performance of the OE blowers but having a bias in the higher rpm ranges where it clearly exceeds the VE of a normal M90, you can actually experience a significant rush of power beyond the normal "surge and die" phenomenon of the OE M90. The MPX is fun to drive. It can put a thrill into higher rpm use. It can also flirt the threshold of maximum cylinder pressure over a much wider rpm range than the S-port or any other OE M90 version and this is more important than most people realize. It's probably as much fun as pump gas can safely allow.
When people ask about the AR they usually don't even know what they are asking, and all too often even when people get them on their cars they find ways to talk about what they gave up compared to their M90. This is just such a contradiction I can't find the words. I guess it's kind of like putting drag radials on your car and then complaining that you can't lay as much rubber as before. It just makes me scratch my head. Over time I've come to realize that if the benefit of the twin screw blower isn't readily apparent such that one has to ask, then it's just not for you. An AR wasn't the first thing I tried, nor should it be anyone else's. You'll know when/if you are ready for that when the time comes (if it does).
It certainly doesn't take an AR to have a fast SC or to have fun in one. But I also noticed a hint of "I want to make xx boost or xxx hp" in the original question, and I just need to point out that regardless of how you make boost or how you make HP, stepping beyond a certain level requires a willingness and ability to go beyond the simple "recipe" grade and take on a higher level of responsibility for the outcome of your efforts.