I've heard of 3.5L conversions with custom sleeves, pistons, and machining. But certainly at some point one exceeds the limitations that the designers were imposing with other design choices they made. Somewhere along the line, you end up going the route that Greg Gordon took with certain kinds of forced induction. That's probably the route I'd recommend for more oomph. I've been considering it for after the Verde goes back on the road and has been there a while.