see i think everyone is mislead by the fact the greddy is a piggy-back. Seeing as the Greddy/Re Rx7 is running stock ecu + ultimate , and has been in the d1 gp for along time without a problem... along with it being the Touge battle Champion in Hot Version (beating a GTR) i dont see it as being limited... its seems to have a few extra function that the pfc doesnt have.
I personal think as tuner have more tuning time with the ecu they will be able to better use its function and take full advantage of its power. I think its pretty much the same go with the HKS ecu, HKS offer alot of ecu (from the fcon to piggy style just like the greddy) which also have alot of function, but untill a tuner becomes familiar with the ecu they wont be able to take full advantage of it... well thats my 2cents
(warning this is just my personal opinion)