I think this could be very useful in the deals/buzz forums in particular, and addresses the major shortcoming of MyFT which misses new threads which you might be interested in.
If software performance is an issue, perhaps consider replacing (or combining) MyFT with a new highlight+hide thread functionality? I.e. when viewing a forum your subscribed threads are highlighted and ignore threads are shaded. I guess this is analogous to viewing emails with stars or priority symbols.
Personally I would want to keep MyFT rather than replace it, as being able to see all new activity on subscribed threads across forums is useful.