With the 13/43 bevel gears my bet is that you'll even have to go down on the clutchbell pinions. Something like 15/19. And even then you're taking the risk to have your engine overheat and underperfom.
The feel of the 10/43 bevel gears is awesome under the trigger and I think it increases the lifespan of transmission components. Without naming any brand I can say that I've seen many GTs needing to replace transmission parts such as bevel gears or CVD pins rather fast. It seems like a really good move from TeamC to have chosen 10/43.
Try even bigger on the clutchbell? Or try a 250$ on-road engine I have no doubt that you will be pleased.
I battled a few laps with a really fast guy driving serpent GT and he did have a bit more top than me. But I had an advantage in the infield, especially in corner exit and sweeper section too. On a 5 min battle I had a slight advantage overall.