A guy in our club had an Enforcer years ago... powered it with an ST 90...

It was fast, but I wouldn't call it extreme... but faster than it would be with whatever .60 you throw at it obviously...
The only way to reach extreme speeds with the Enforcer is to get the thing up high and then do the longest power dive ever... don't forget to pull up
That guy in our club with the .90 on it loved to do this... but he also lost his plane to flutter while doing it... I guess he reached extreme speeds...

All in all it's a nice flying plane... the only thing I didn't like about the way it flew was that "wobble" it seemed to have when it was banked... the plane always seemed to be shaking its tail, and when banking that shows...

O, and they're LOUD... that prop running through the wing sounds like a siren...