Just a heads-up: As Sam and others said, you're limited to 2600 rpm with that prop, so you'll never see 75% at altitude. If your %'s are from your EFIS, it's lying to you (or more likely, computing based on being lied to). You can't make 75% power at the 8k' altitude ranges without turning *the full rated rpm of the engine* (2700rpm). Because the prop is *restricted by the mfgr* to 2600 rpm, you can't make 75% at that cruise altitude.
I'd suggest calling Van's & try to get them to tell you expected cruise performance *with your prop*, *at [x] altitude*, full throttle, leaned, at the rpm you're seeing. Include your MAP at that altitude; if you have any kind of intake restriction they'll be able to see the discrepancy from expected pressure at that altitude.