I am not sure a new bike is going to give you 3mph of speed, that is on you (the engine). You could drop maybe 2 or 3+ pounds off your bike weight but that is a good poop and a water bottle. Getting a new bike is always a good plan LOL, but don't think there is going to be some miraculous speed increase. So looking at something more aero and lighter with optimal gearing is probably only going to be limited by your budget and comfort in position on a racier bike, the point of fitting, it would seem to me as someone whose DH does the tweaks and has bought bikes online, is not to retrofit a badly sized bike but to ensure the bike you are going to buy fits you and the is the geo you want. So if you are generally clueless you want to find a really reputable bike store to do that before you buy, and speak with the person who fit your old bike for advice, you already paid for that? .