Engine(s) performance at altitude

[kylant]

looking at boats with these engines:
5.0
Excalibur 330 and 343.

wondering if anyone share real world experiences with any of these engines at altitude. we are at 6500 ft.
curious about both v-drive and direct drive configurations.

we have had a GT40, ZR4, and H6.
the ZR4 in the v-drive was a dog, the others were great.

thanks for any insight

[kylant]

anyone have experience with these engines at altitude?
thanks