Ken D-
From what I know you can't play midifiles/sequences on flash cards like you can in the Alesis QS series. This is a great feature about those machines. The Yamaha S80 (which I also like very much!!) allows you to play sequences off cards in the same way.
No you can't import your own samples into the XV88. However, the XV5080 modules allows you to expand it up to 128MB of ram and you can load in Wav, AIFF, Roland and Akai samples. If you don't want the 88 note controller, perhaps you could get one of these and use your Alesis (with it's nice unweighted action) as a controller.
Sonically, you would get fantastic results by layering up the Alesis and Roland sounds. I used to own an S4plus (which basically sounds the same as the QS synthes) and when I layered it with my Korg and Roland stuff, it was awesome!!!!
However, do also check out the Yamaha S80. This is slightly cheaper than the XV88 and allows you to add different plug in boards for virtual analogue, harony processing etc. The Roland boards allow for additional sample ROM. Compare both the S80 and XV88 and decide for yourself but IMHO these are very closely matched (although I have a slight bias towards the Roland sounds! This only IMHO I might add!!!!).
Yes we synthe players are the real big winners at the moment with all this great new gear coming out! Mabey I'll start playing lotto so I can buy even more!!!!! (I'm poor afterall!!). Cheers.