Rotax 915iS + Extra weight to change CG

[hk1232]

Someone tell me why the Rotax 915iS in an RV-9 (like the Lockwood demo) always propose extending the forward position rather than adding weight to align more closely with the heavier Lycoming.

I understand lighter is better, but if there are downsides of the lighter engine isn't this a reasonable approach to normalizing the CG?

The Rotax is 185 lbs and a Lycoming 320 is 244 lbs. A 2" x 8" lead disc would come close to zeroing it out. I haven't done any FW forward work yet but that doesn't seem like it would be a space issue.