Some people can go very fast with small fins, I did my highest speeds with an 30cm tectonics falcon f1 on a Is105. If you are not the power style surfer you could use this fin. This fin gives somhow a lot of acceleration @ 110-120 degrees downwind
If you like power on the fin I would go for the Select sl7 33cm, this fin is really fast deeeeeep downwind.
2 different styles, both very fast. 30cm doesn't work in no-wind, 33cm is easier to control. >33cm fins will be harder to ride on tight courses in strong winds. Only very experienced/ strong/heavy surfers can handle that.
Lots of deboichet fans around here, sorry I am no deboichet fan since I am confinced that select & tectonics deliver at least the same performance. Albeau used tectonics falcon f1 on fuerte/karphatos speed world cup (on videos & pics...)
Back on-topic ====> 28cm sounds to small... Don't go smaller!!!