Well you'll need a board that floats and that has a considerable volume to tolerate the 8.5 sail. So i would suggest something in the area between 120 and 135 L because these boards usually tollerate a range between 6.0 and 9.0. Also, you clearly are going fast and want to go faster so taking that in account, the fact that your an intermediate and that you want to carve it sounds like you're a candidate for the SB Furuta, or, if you want the GO. These are freeride/slalom boards which focus mainly on jibing and alround fun with a lot of comfort, easy use and just pure blasting on the water.
For the second question: carbon masts are lighter then non-carbon masts and are much more responsive therefore. Also, it is often true that newer sails are easier to handle because of the new materials that are used. I don't know when your 6.4 was built but it can be a bit of both as you say, that is giving you problems waterstarting. However most sailors will tell you that it is probably the mast you are using that is giving you problems, if you'd rig the 6.4 with a 55% or > carbon mast you'll see that is much faster and much easier to handle.
I'll leave Roger to tell you how to get to the next stage, but what i did, i just rampaged this forum :d
Haiko, AKA crazychemical