pic taken from another thread:
here are the relative lumens @ 350mA
P3 73.9-80.6
P4 80.6-87.4
Q2 87.4-93.9
Q3 93.9-100.4
Q4 100.4-107.0
Q5 107.0-113.6
at 1A it shouldn't be too much of a difference either since they are very close.
remember the luxeon bins had about a 20lumen gap inbetween them and they were barely noticable.
Luxeon
T-bin 67.2 87.4
U-bin 87.4 113.6
here is a link to a review of the fenix LOD-CE Q2 and in it they compare relative brightness to their earlier review of the LOD-CE P4 and in this case the P4 version was just slightly brighter.
http://64.233.179.104/translate_c?h...rch?q=millermods+cree+arc&hl=en&safe=off&sa=G
i am no expert here, but changing from a P4 to a Q2 would only make sense if you wanted a different tint.
EDIT: the review is in another language but if you scroll down you can see the numbers LOD-CE P4 being(787,742) and Q2 being (760,943)