Reporting something I find interesting and wonder if other AR members have found the same.
Currently I am working up another load for my .243 W because components are not available for my original load which shot really well.
I narrowed powder choice down to AR2209 ( H4350 ) with Norma cases, Hornady 95 gn SST and some old Norma Superflash Primers I want to use up. The most accurate load gives me 3083 fps av.
I have tried exactly the same load with CCI 200 Primers which shoots slower and nowhere near as accurate. And, from this assumed the Norma Superflash Primers burn hotter resulting in the better speed / accuracy result. So, with curiosity decided to try exactly the same load with Federal 215 Magnum Primers as these are definitely hotter burning and I use these great primers a lot in several magnum and non -magnum loads in other calibres. Lo and behold, what a difference in mv ! Av. fps is 3242, a difference of 159 fps ! OK, accuracy is not as good but indications are that a little load tinkering will fix that. Best of all is despite this higher mv there is absolutely no indication of higher pressures. I am both amazed and excited about the possibilities for further development for this load.
I admit I have noted similar affects on mv with other loads also when substituting the 215 Magnum Primer although often with accuracy loss. But I think if accuracy / pressure levels are unaffected it's a great way to achieve a slighty flatter trajectory at virtually no extra financial cost.
Would welcome comments from others who have findings of a similar nature.

I've never experienced anything as dramatic as your results.
Most of my primer changes have shown little or no variations, I get bigger changes by switching to different brand brass.

One wonders if perhaps you failed to mic your cases and they may need trimming..thats a lot of velocity as a rule for a primer change unless your old primers were a bit cool or old. I use Fed. 215s in all my magnum calibers and 210s in non magnum, when switching I have never noticed more than around 25 to perhaps 50 FPS increase with the 215s and sometimes a little less than the 210s, why these things happen is up for grabs IMO, nobody knows.