Latest CTT advice is that 20mph limits must be observed (which, in practice, means courses going through them will likely be retired). The rationale, and I understand this, is that it does not look good, nor is it safe, to have bikes overtaking cars observing the limit - unlikely though the latter is.
Before anyone suggests 20mph is fast in a bike, it really isn't in a TT - to achieve "evens" and not embarrass yourself, you need to be cruising at quite a bit over 20mph to compensate for the start, turns, and junctions. The better club riders will be cruising in the high 20s, even in a casual evening "10".
It does look as if the future of TTs will mainly be 70mph dragstrips, if you're feeling lucky enough.