There can be little doubt that there will be some increased vibration with solid motor mounts. But since most people have been driving their T-bird with crushed mounts anyways, it often doesn't add any vibration that people can feel. Since the SC engine is balanced internally, it does fairly well.
I also purchased my mounts from MN12Performance and had good service.
Keep in mind that the solid mounts, in general, cause the engine to sit slightly higher. This has been a surprise for some people that had raised SC tops that were close to the hood before the mount change. I didn't have any troubles on my car as my top was still stock. And most raised tops sold today won't rub either.
SuperCoupePerformance sells some solid mounts that are supposed to result in the same engine mounting height as the stock fluid filled mounts. For those with a clearance concern, they are a great option.