I honestly think the JP-2 or 4 would be best for that particular engine. The 3 will make the engine feel torquier but it will choke it on top end. It will feel stronger all around on the 2 or 4.
The Dynamite Platinum series pipes are cheaper, lighter and 3X stronger than the JP pipes. You could try the Losi sport or 007 exhaust or the High Torque or High Speed version. They all are good pipes. That Force .28 is not a reliable engine, I'd save for a new .21 engine with a matching exhaust. You'll save on gas in the long run, plus any of the newer .21's will SMOKE that force .28.
Get off your butt and try the JP-2, you'll love it.