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I'm wondering if slacker head tube, heavier inbetweener bike like Stanton and fatter rubber would make me better or just make me slower? Going slower might stop me breaking myself but it messes up some bits where you need speed to get through sections clean.
You can put fast rolling tyres on something fairly slack - I have Maxxis Minion "Semi-Slick" on my hardtail. The head angle won't dictate how fast you go, that's all about line choice, technique and (on the flat or uphill) power and fitness.
Line choice is the biggest thing that you can do to adjust your speed across all terrains, obv's massive power is going to make the biggest difference to flat land and (non-technical) climbing.
As an only MTB? Yes, I think he'd have more fun on a more versatile bike, if it was his only bike.
I've got some options available to me, however.