I suppose some are replacing the engine because they're sick of trying to deal with the 2.5, and some just want more power. When running properly, I think the 2.5 provides adequate power for the Maxx. However, I'm a power and speed junky. I'm really looking forward to the release of Fantom's new .18. Just like the Sirio .18, it's a direct drop-in replacement engine, but it should be quite a bit less expensive than the Sirio.