Recently put a 1 watt dropin from Lighthound in my old P3, it works great with a rechargable 123 cell. Output is just below the max output from my HDS 42, very close. Tint on the HDS is much better, dropin has green hue but i don't mind it...good little light.
With a primary 123 cell output is much dimmer, just below the second brightest level on my HDS. Tint of dropin is greener with primary cell as well.
I had to sand down the reflector a fair amount to get bezel to screw down all the way on my Surefire, i've read about this issue before with these dropins, seems they could get the length right from the manufacturer, oh well. Also no polarity protection so don't put the cell in backwards or poof bye-bye led. Overall I'm happy with it, good 'low key' take to work light. With an old G2 tailcap i can easily turn it on one handed as that cap turns much more easily than my stock P3 tailcap. Gets quite warm in extended use so hold it tight..., i got a good hour at almost full output with rechargable and got bored with 'runtime test'.....