I always found the ratings the DSLR screens to be rather optimistic in terms of what speed lens you can manually focus well with. The stock screens, IMO, make it tricky to really nail things with not just stuff that is f/1.4 or f/2 but also f/2.8 and even for f/4 I find they help tremendously (even if things get darker). The stock DSLR screens are so poor for focusing, I almost feel like you don't want to trust them unless you are like f/5.6! They make things across the frame look waymore in focus than the actual photos.
Thanks for the feedback! Usually with the MP-E 65mm, I'm shooting in Live View, which obviates the focusing screen. But sometimes I use the VF, and I have the Eg-S screen for my 5DII - so, I'll have to give it a try for macro with my f/2.8 lenses (MP-E 65mm and 100mm f/2.8L
Macro IS), and maybe with something like the 70-200mm f/2.8L
IS II, although AF usually does fine with that lens.