TalkTalk support looks to be pretty ropey from what we see here, but my brother hasn't needed to contact them. How good his speeds are I don't know - just after connection he was getting mid-5000s kbps, which was around what TT sales forecast. Whether it slumps dramatically is unknown, as it is for 90% of users.
He won't be doing the same amount of upload/download as you, but a total of 6-10GB a month is about in the middle of the "allowance packages" on offer from most ISPs. As the TT allowance I think is 40GB I can't see they would target you as a heavy user, so any degradation would be down to lack of capacity. I would imagine any shortages are being addressed - a lot of the problem was the dramatic take-up in the early days. They won't want to get a long-term bad reputation.
But it isn't me that is looking at an 18-month(?) contract if it isn't satisfactory.
I would certainly steer clear of Tiscali though - the whole setup seems dire.