The shorter the a/c range, the shorter the loitering time over target, that means less fighters over target at a time. So P38 should be more effective at LRCAP 4 hexes away than P40 for example.
This is definitely reflected in the game.
Maybe what I'm seeing is WAD, maybe not. But it seems that beoynd sertain (IMO rather short) range LRCAP have no effect at all vs. transport missions.
Here are some results of my test using Guadalcanal scenario:
First Kittyhawks (normal range 5), two squadron flying LPCAP, 80 CAP 20 rest. Japan have 7 Mavis, 2 Emily and 31 Betty flying supply transport mission:
LRCAP over Buna, distance from Port Moresby 2 hex.
Number of kills (6 turns): 0,0,0,0,0,0 (none)
LRCAP over hex 99:130, distance 1 hex
Number of kills (5 turns): 6,6,8,3,3 (=26)
Then P-39 (normal range 6) two squadrons. Same settings as previously
LRCAP over Salamaua (distance 3 hex)
Number of kills (5 turns): 0,0,0,0,0 (none)
LRCAP over Buna (2 hex)
Kills (5 turns): 4,5,2,2,2 (=15)
LRCAP over 99:130 (1 hex)
Kills (5 turns): 8,7,3,6,1 (=25)
Like I previously said, A6M2 Zeros (normal range 9) could shot down tranports when distance was 4 hex (from Lae to Port Moresby). A6M3 Zero (normal range 7) could not.
I attached my "head to head" test save file (.txt -> .pws), if someone is interested. At the moment japanese planes are flying supply mission from Rabaul to Salamaua. Just set allied fighters to fly LRCAP and see what happens.