Welcome to the world of the P60s! There are sooooo many mods for the P60 compatible Surefires that it's impossible to count!
Drop in mods: There are hundreds of P60 drop ins to throw from. The best throwers are XP-C pills behind a aspheric lens. The highest lumen output P60 drop ins are the quad XP-Gs, they output around 1700 OTF lumens and are available on the custom B/S/T and are made by either VanIsleDSM or Kerberos on the custom B/S/T. For all the others, i recommend you to PM either Dave (Nailbender) or Wayne (Vinhnyguyen) as their work is much better than the genric LED drop ins you find on ebay or elsewhere. They will be available to accomodate any needs for output levels, tint, reflector choice, voltage range etc. Both of them have got sales threads on the custom B/S/T forum here, where they can be contacted.
Body mods: The first thing i will recommend you to do would be to bore it out to fit 18mm cells like 18650s, 18350s or 18500s(with a A19 extender). It would give you a 30-40% increase in runtime over CR123 cells, are free (being rechargeable) and have higher max current capabilities (10A max). There are three CPFers that would provide this service. They are precisonworks, bugsy714 and Electronguru(Oveready) and they've all got threads somewhere in the custom B/S/T. Of course there are other custom bodies for Surefires as well. Fivemega makes AA and CR123 or 3 AAA sized tubes. So you can basically run your 6P with any cell choice, from 1 CR123, 2 CR123, 1 17670, 1 18650, 2 18500, 2 18350, 2AA, 3 AAA, as long as you've got the right drop in for the task. Most P60 drop ins are 7135 based, which means an input range from 2.7V to 6V, so it is only compatible with 2 CR123s or 1 18650 but you can request for a wider voltage range.
Bezel mods: You can change pretty much anything you like, from the bezel ring to the lens. An ultra clear lens will provide 5% more output for free over the stock borofloat glass lens. There are lots of heatsinking heads to choose from as well, from the Cryos cooling bezel to Fivemega's beefy heatsink, which would allow for higher output drop ins to be used. Both are available on the custom BST with a search.
Tailcap mods: You can use a Delrin shroud for tailstanding capability or a Moodo Triad tailcap. A Diamondback tailcap provides better grip. As for the switches, the stock Z41 tailcap twisty is only rated for 2.5A at 9V so you will need to change the switch if you intend on running higher output drop ins (a quad XPG runs at 5.6A). There are many choices...
1. McClicky: Very good and my personal choice. The max rating is 4A (but will not melt until after 7A) at 12V DC for 10 000 actuations. The resistance (as measured by Dan at Oveready) is 92% of a ZeroRez tailcap. You can get Soft, med or hard press boots easily.
2. Kroll Tailcap: Not made anymore but you can still get them at TheSandwichShoppe (i can link you if you wish). Some people prefer the shape (conical), as it is easier to actuate.
3. Netkidz: Not made anymore but you might be able to snatch it up on the B/S/T. An alternative to the McClicky.
4. FETtie: Made by VanIsleDSM in the custom B/S/T. This is the most advanced out of all of them, as it is essentially like a relay (think of a solenoid in your car) that can easily handle 400W of power or 20+ Amps without breaking into a sweat. The resistance is the lowest out of all of them so you WILL get more light in direct drive setups and more runtime in regulated setups. Available in both forward or reverse configurations.
5. Stock Z58: Is a clicky, but not recommended for higher powered drop ins like Quads/trips/XMLs as the max current rating is 2.5A. Higher resistance than a McClicky and no room for changing boots etc.
And lastly if you get bored of the black colour, a Surefire is one of the easiest to cerakote as the innards come off easily. You can coat it in anything from white to orange, here are a couple of examples borrowed from Gingseng
The P60 world is a very good gateway into the world of customs, which together with high output HIDs every CPFer would eventually be obessed with.