I can't comment on the 3D props, but suffice to say that the IMAC precision flying sequences have a lot of similarities with pattern. With the Extra being a somewhat draggy airframe, perhaps an 18x8 or 17x10 for IMAC precision (??).

I was able to add another prop to the list I've tried - the APC 17x12N (narrow) pattern prop - and, as I have said about some other props, it isn't horrible but it in no way matches the brilliant performance of the 15.5x12W with my [link=http://www.rcuniverse.com/forum/fb.asp?m=9732410]SAP-180 cookbook[/link] set-up.