Thanks for your interest. I have used the Max 4 FFTease externals in Max 5 and have not experienced any problems. Rest assured that I have plans for future development of FFTease. However I will not be able to take advantage of any Max 5 features until the new SDK is released.
Are you referring to convolution with an impulse response? Can't do that in FFTease. However I have written a LyonPotpourri external called convolver~ that will convolve a source sound with an arbitrarily long impulse response. The only catch is that the convolution is done in non-real-time. convolver~ has not yet been released but will be part of the next LyonPotpourri, which will be made available sometime after the Max 5 SDK hits the streets.
It would require a partial convolution algorithm. I've heard that some of these algorithms have been kept out of the public domain by patents, but have not looked into this too closely. CPU limitations should not be a factor, since even with fairly long impulse responses, convolver~ computes quite a bit faster than real-time. The main reason is that I haven't yet felt the need for a live convolver in my own work, but I can see how it might be quite useful.
As I understand it, the impulse response is chopped into smaller pieces and then used for multiple overlapping convolutions with appropriate delays. The point is to reduce overall latency to less than the size of the impulse response, which would be problematic for real-time reverb generation.
Thanks for the kind words, Bertrand. There are new versions of LyonPotpourri and FFTease in the works. However everything is on hold until my Max/MSP programming book comes out. Hopefully much or all of this will be out within 2011 but it's not possible to give specific dates.