I have a very simple approach to this:
- If your Amiga has a graphics card, use Cosmos' library
- If your Amiga uses AGA/ECS, use Fblit
Simple? Done! At least it's what gives me the best results, and I'm combining these with multiple other patches, as well. I just wish Cosmos were still releasing public updates to his library, but I understand having to lock it down due to people trying to profit off his work. I still scrounge the net to look for the latest versions, though! 
if you've a GFX Card and this one have a blitter isn't the P96 or CGX supposedly use that in place of the Amiga Blitter freeing the CPU for other things?
The advantage of Blitter is freeing the CPU if you replace Blitter functions by CPU routines then you loose that CPU. If the CPU is doing nothing that fine but if you render an image for instance in the background and using you Amiga aren't you loosing time? All in all for GFX Less Amiga a combination of the too would still be faster but it can be tricky to balance the too not knowing what is happening in your system. I suppose that some kinds of settings depending on CPU power and RAM speed could be done to use the too in conjunction for maximum perf.
Kamelito
