Yes, I completely agree, a longboard or transition board like the '07 Phantom 301 or the 2008 Rio M or Rio S would be much better for you in < 10 knots of wind.
If you get a Phantom, I'd look for the 301 cm long version. I had one of these in 2007 and it was pretty good. Here's a link:
I like the 2008 Rio M even better and it's about the best all around transition board I think any one has ever designed.
The 57 cm Clipper box centerboard works very well to allow you to sail upwind in very light winds, but when you stow the centerboard and put in a larger fin (approx. 56-60 cm) the Rio really give very good shortboard like performance.
At your weight, you could easily sail the Rio S, if you can find one.
Here's a link:
So, yes look for a Phantom or Rio for your light wind sailing.
Hope this helps,