Depends on the wind.
I sail my 25 cb with the board pennant down about 1 foot in light wind. I think the center board trunk works very well in light wind.
I rarely fully lower it. I find that in a heavy wind that varies somewhat, I allow the boat to round up until it holds a constant heel then gently follow the tell-tales (tails) to keep the best trim. That is with about 3/4 of the penant out. It works great for me but I don't know if I explained it well enough.
I have been lazy enough at times to not lower the centerboard at all and still had some great 15 knot days.
So, If I were you, I would give it a try at different positions and note the proformance. If you are in shallow water, I don't know about the 28, but the 25's rudder drafts 2" more than the centerboard trunk and that is with out a load in the cockpit. I have grounded my rudder

once and don't plan on doing it again...
sail hard and soon, the water is getting hard!
r.w.landau