Pressure rise.
A faster pressure rise theoretically burns the powder more efficiently, taking advantage of the energy stored in the powder earlier in the pressure curve. It also can help with accuracy. A lot of BR shooters seem to subscribe to this idea.
With slower burning powders this has generally been my experience too. Not every time but usually. Went through this last year with RL-26 while messing with LCD dies. Bought LCD dies for 4 different cartridges and none of them would size the neck enough to hold the bullet. I could pull the bullet out with my fingers. So happened I bought extra standard diameter mandrels in 270. While waiting for undersize mandrels to arrive I sanded down one of the spare 270 mandrels. More than I intended........ended up at .2727 according to my mic. What the heck, I'll give it a try. I was already making straight ammo with my old method, but the tight necks (which I like for a hunting round anyway) produced more velocity and better accuracy.
Also, I will almost always get a velocity drop from new brass to fired brass, most likely because of the higher volume of fired brass creating less pressure rise.
Although 200fps is a lot, and I don't think it all can be attributed to less neck tension, especially because you did crimp the ammo.
