I shoot competitively in the Army (High Power, Combat and Sniper), as well civilian tactical rifle, and IPSC (Production[which I suck at]).
Honestly I don't do anything special for diet. Even for some comps that are physically demanding (running w/ water cans, ruck march then shooting out 1k) I just eat what I always eat.
For static shooting (HP, Tactical, etc.) my heart doesn't rise. I wore a heart monitor (EKG) a few times to see what happens, and my heart rate usually drops 5-10 bpm during firing. For "run and gun" matches it climbs up based on output level, then drops slightly during firing. The assumption is this is due to my breathing cycles and focus/meditation techniques.
Best thing I can tell you to help your scores, is take a yoga class. I do my dry fire drills in my head when I'm in my yoga class =).
"Go For Broke"
Fat Kine-230/24% @ 6'2"