Hello,
I don't no what Genesys did tell you but we have 81c with symposium express package 2.5 and 3.0 and both they run with Genesys 6.5.
It runs fine ad our side.
So don't let them tell you anything, and there are not many features more that you can use, if you take MLS 4.2, in combination with Genesys TLibary.
Even if you use the meridian link module in the switch its even better because its redudant en the symposium is'nt.
greatings marco