My personal interpretation of a grenade motor is something that makes best power from hours 2-4 and is pretty much done by 10-12 hrs. This, road racing duty cycle. For autocross, that would be more like 4hrs total service life. Some types of engines can tolerate stuff like thin seats, loose bearing clearances, 0w5wt. The BP's less so it seems. I think this stems from the genome of the B series Mazda. Just never intended to make big power or rev high, even as the original 1.3L GLC motor it came from. The bottom end seem to be quite robust if kept below about 8200 sustained and 8500 transient. Valve trains need big seats and 75+ lbs seat pressure. Cam profile is key. Lash ramp to face transition and nose profile all affect peak acceleration and play into valve seat wear. If we pull a motor down and lash is good, seats are always good. Lash bad, seats bad. That tells me lifter, journal, cam wear isn't an issue. Although the cam journals are second to go when they lose oil pressure. Anyway, that means throw seat pressure at it to keep them from bouncing, as the BP6D's are won't to do.

These days, we find building for 350bhp+ and getting 50hrs out of it is not difficult. Forged everything, SS & Inconel valves, keep it below 8200, double valve springs, billet pump gears and damper, 10w40 Synthetic race oil kept below 240° and coolant kept below 215°. The XX rings we like don't make the most power or run coolest but they do keep blow-by in check when running a basically stock wet sump.

Sounds like excess revs causing the problems here mainly.

I was at 62lbs of seat pressure with 11psi of boost but on custom cams. My problems pointed towards weak valve springs.

If my math for valve area is correct you are loosing 28-33lbs of spring pressure due to boost. That does not leave you much pressure to close the valve. Seems suspect to me.

I too had the same issue with the seating face of the valve concaving from a narrow seat surface. I widened the seat and went to the light double springs from the singles. Don't have any days on the engine since then, but well be keeping a close eye on it. I never had any issues running the stock valves though with the stock seat grind. If this don't work I am going to go to my back up head and use stock intake valves with the oversized ST inconel exhaust valves.