There's a couple of very good sites here that do mods. To name a few, cartmods.com, cart-lab.com, firetanksoftware.com and more.
These sites are all great and I personally would highly recommend getting with lildawg (xcartpros.com). He's very fair, honest, quick and extremely knowledgeable.
Xcart took forever on my upgrades as well so I stopped having them upgrade our store. You can also ask for a refund and they'll do it if they haven't started it yet.