The pre-MY09 Leopards are essentially identical as the MY09 Leopards in terms of power and performance. However, the MY09 is a pretty good upgrade in terms of reliability with the added 3rd bearing support and the slightly beefed up bottom end.
I remember during the transition years from pre-MY09 to MY09, we found the MY09 liked to rev about 200-400 RPM less or so in most cases.
I think you’ll have a better and easier time if you can find an MY09 engine, however, you may never have an issue with an older one either if you take care of it and aren’t screaming the RPMs.
Compared to a Rotax, the Leopard of any model year will beat the Rotax on tracks with tight corners and long straights, but on a more flowing track, the Rotax might have the edge, as it makes all it’s power in the mid-range. At New Castle, a few people (Di Leo and Rios) were able to make the Rotax work against the Leopards at a national level, but it wasn’t easy.