From the point of 0ms to 30ms (percussive sounds) and 0ms to 50ms (fluttering sounds) you have to deal with Haas effect (more delay = more stereo widening) and comb filtering (more delay = more filtering from bass frequencies to high frequencies)

OK thanks I'll try listening for width changes and see how it goes

For the really short ones of just a few ms I have to go by the comb filtering (still a bit of a guess at this point if it's comparing say 2ms and 6ms). Around 20ms I listen to the transients - that initial click sounds 'thicker' to me.

Is comb filtering similar to when mics are out of phase and the low end kinda disappears?

Comb filtering is a phase issue when two sinus are out of phase of a certain delay and a filter of loss in dB appears from the fundamental to the harmonics frequencies. F1=1/2*Delay F2=3/2*Delay F3=5/...

"A less severe form of comb filtering occurs when the outputs from two microphones set up at different distances from a sound source are combined — a situation familiar to anyone who has miked up a drum kit, for example. Because the more distant mic receives less level than the close mic, the depth of the filtering isn't as pronounced as in our flanger example, but it can still compromise the overall sound."
Guess so, thanks