stock XR's & no drive showers from memory when I saw the boat a few days ago.

Yes, correct. Stock lowers, no showers.

Apparently, some cavitation is normal with this setup. I have run the boat 5-6 times now, and am able to get on plane with minimal slippage by lowering tabs fully and tucking in drive. Tabs full up as soon as bow drops, then dive up to 2-3. Takes a long time to plane (compared to old 27 Baja), but runs great once up. Thanks for the help!

There is what has been refereed to as the "celery stick" at the mounting point of the trim rams to drives.
This can be rotated 180 around to the front of the pin which lets the drives tuck further under. Helped my friends 382 plane faster. Has showers also which i think add to the cavitation out of the hole.
Tabs at about 2 was about all it would like.

# 37... Ok the real name is "Trim Adjustment Insert".

Any luck?

You really should not need to make mechanical trim/tab mods unless you are running a race boat. For some time, I ran the bravo 30p props which were stock on my 99 353 w/hp 500s / bravos / boxes and the tabs were not needed at all to get on plane. I have tested with 28p 3 blades and the nose ran higher but the blowout/planing became a problem. Once you move to a more top end prop you will use your tabs. As other members have suggested, your props may have been significantly altered so you could be running a very different profile than you think and the blowout indicates too much slip/air/turbulance around the props (or over throttling). As a matter of best efficiency, you should always launch with the drives fully in. See if you can get a set of stock props for your year and start from there. Then work with a respected prop shop or Merc racing for the necessary changes (if any are really needed). Some mods may only yield 1-2 mph but deliver all kinds of undesireable launch and midrange characteristics - your choice. Lastly, if you are planning on any significant horsepower upgrades, your modified props may no longer be optimal as it sounds like they are now. Good luck and keep us posted!