The maximum upload speed for a dialup modem depends on the modem standard used:
V.90 - 33600 bps V.92 - 48000 bps
I think the reason they insisted you get the professional installation was because the box you get now is a transceiver - and the microwaves it puts out are quite hazardous to your health. If this is the case, you could get upload speeds close or equal to your download speed, but the latency would kill you - so it wouldn't matter.
-- Tigger "It's not the years, it's the mileage" ~Indiana Jones