It just depends on what you want to achieve. At your level you certainly won't be competing to win anything so the TT's will be more for personal achievement goals (which is what they are very good for). If you want to do that purely by fitness and position on the bike etc, that is fine. If you want to help it along a bit by buying some speed (wheels, helmets, frames etc etc), the decision is up to you. I would suggest you at least do a few TT's before throwing alot of money at it.