OK, I'm home again after work and can answer my own question!
Like jkinkennon says, the buffer size can be reduced dramatically with ASIO drivers, or, as if by magic, using ASIO4ALL. In my case I downloaded and installed this and after I figured out how to configure the outputs in Advanced mode I have managed to configure Hauptwerk to use a buffer size of just 256 samples which translates to 6ms or less. Now I don't notice any sluggishness at all when playing!
ASIO4ALL (free!) is here:
http://www.asio4all.com/Now I just need to find myself a (some!) better keyboard(s)!
Cheers,
Colin