I don't understand why the old K&b 61 could not be ported for 13x5 props too, that's a good prop for the HP 61 PDP which is based on it. Maybe it's just that Mr. Lee only does porting for high rpm applications. Apparently the Twister is not better on small props, but it is better on large props.
Yes, the Sportster can turn a 13x5, but not nearly as well as a Twister 61, and it is much heavier. I hoped my K&B 61 could be made to run more like a Twister, but apparently that is not what is offered.
Jim