If you have a shorter shield right now, have you tried lower wind deflectors? Memphis Shades sells them. I had buffeting with my batwing until I installed them. I had a switchblade 2up before the batwing, although the buffeting wasn't as bad, it was still there (I didn't get the deflectors until after I switched to the batwing).
A test, is to put your hand down by the tank (in front on either side) while riding to see if wind is coming up in front of the tank on either side, below the shield.
If this is your problem, the tallest shield out there will not solve the buffeting, a lower deflector most likely will.
You would need the kit for exposed forks and add the mounting kit:
Lowers for 2004 - 2009 VTX1300C exposed forks (memphisshades.com)
You can see them on my bike: