You’d need to bung quite a bit of extra oil in to cause them to not achieve full travel, boxxers don’t behave like LT single crown forks as they have that bit more internal volume…
12.5 stone you say, have you considered using a soft spring with some pre-load washers added, just to soften the fork a bit?
Oh and is the fork rebounding quickly, or are you finding it packs down a bit on steeps/bumpy rooty stuff?
That might explain things a bit if it’s not extending sufficiently between successive hits then it may well be not working as intended…
35-40 PSI seems quite a lot of pressure to me on a DH bike given your weight, with or without tubes you shouldn’t be finding it easy to pinch flat… I’m about a stone heavier and don’t go above 30psi at either end, but YMMV of course.
I’ll admit to generally preferring a softer fork and a softer tyre, but with relatively quick rebound to keep the tyre tracking the ground and gripping, but that’s not to everyone’s tastes and yes on bigger impacts or if really grunting on the pedals you will blow through your travel there’s a limit to what compression damping can do IME…