SC, not suprised you experienced little difference in recoil between the 10 and the .40, or about the "light show" with the Remington-UMC ammo. I've done a fair amount of chronographing of factory and reloaded 10MM since my first 10MM, a Bren Ten back around '83-84, IIRC. The Rem-UMC load is one I have not chronographed, but I would be surprsed if it got anywhere near 1150 FPS now days. Most factory 10MM ammo of the last ~20 years seems to be drawing closer to duplicating .40 S&W ballistics, than what the 10MM cartridge is safely capable of. With some modern factory ammo, I can see why some shooters wonder why anybody would bother with 10MM, since ballistics are about the same as published .40 S&W. I have shot a fair amount of the Remington .45 185 grain JHP non-+P and 9MM 115 grain JHP +P over the last dozen years or so and find that both give a very bright flash in low light, similar to what you have described. Not such a good thing for duty or carry ammo I think...
I'm still a fan of the "real" 10MM and after the Ben Ten had Glock, Colt, Kimber, S&W( semi-auto and revolver), Ruger Blackhawk, etc. 10MMs. While I am a died-in-the-wool 1911 guy, I have come to prefer the S&Ws in 10MM over the 1911 types. The S&Ws, with their greater slide mass seem to handle the real 10MM ammo with boring reliability, but without Warp 9.5 slide velocities, heavy recoil springs, buffers, etc. of the 1911 10MM platform.
Ps, "Real" 10MM to me means loads like Norma's original 200 grains at a chronographed ( in 5" bbl.) 1200+ FPS, Norma 170 at ~1300 FPS, Buffalo Bore 180 JHP at 1380 FPS, Winchester's 175 Silver Tip at 1270FPS, etc. or similar reloads...ymmv
COTEP 640, NRA Life