A block, by itself is worth horsepower in a few aspects.
Aluminum blocks are all sleeved, which creates a much straighter cylinder with more involved capillary action. Since it's aluminum, which is weaker than cast iron, more material can be added to strengthen the block without making it obnoxiously heavy. This adds to the effect of keeping a cylinder in round. The straighter the cylinder and the less it expands, the tighter piston-to-wall you can run. This translates into a much better ring seal and gained overall cylinder pressures.
Most importantly, a larger bore will be available. Bore has a huge effect of how a cylinder head flows. With a factory SC head, you will gain within the vicinity of 8-15cfm. Bore size also allows for more RPM to be dealt with. RPM is equally significant to gained HP at a higher RPM.
If anyone is scared to use a factory block, filling it will solve your worries to a certain point. Otherwise, an aftermarket block with a larger bore has many advantages over durability.