1. If you plan to change to LED, take a look at this >> Surefire & Li-ION batteries. While you are there, sign-up an account and post any questions you may have.  Oh, and yes, we do recommend a metal bezel when running high power LED to get better heat dissipation. If you decides to stick to Xenon incan, to use rechargeable, you are advise to go for 9V incan as these LiIon/LifePO4 rechargeables are 4.2V/3.6V when fully charged, using 2 of these will be 8.4V/7.2V and will flash (burn) your 6V xenon bulb... the same goes for LED, so check the Voltage range of your LED drop-in before use.
2. you may wanna take a look at TK41 if you are looking for brightness and latest LED emitter, tho TK45 is no doubt with a more unique look. 

more often than not, the brightness different are not significant, assuming when you say 3.6V, you are referring to using LiFePO4 cells.

do note that main factors being:
1. what battery chemistry, eg LiIon, IMR (LiMN) or LiFePO4
2. current draw of your 9V incan bulb
3. size of battery, which normally limited by the size of host
the combination of the above will affect your runtime and brightness

if this is your first venture into the rechargeable Lithium batteries realm, take note of the following too:

* different charger or charging setup required to charge different battery chemistry

Some of us like the LiFePO4 as it's safer. I do not use LiFePO4 due to lower nominal Voltage and lesser capacity. I like running my incan on Li-Ion or IMR, and some of the high power incan setup are "safer" to run on IMR only. 

yes, if you want to run high power LED (anything more than 200 OTF lumens), then it's better to use metal bezel for better heat dissipations (for discussions of how far we go to get a better heat dissipation, search in The Bright Side forums for some tips). if you want to remain the original nitrolon bezel, there are options like lower output but longer runtime P60 LED drop-in for G2 (like Malkoff M61L/M61LL), multi-mode P60 LED drop-in where you can choose lower output for normal usage and use the high output for short burst (like Solarforce range of drop-ins)... of coz, all these drop-in works very well in metal/Aluminum bezel too.

the SureFire G2 can't be bored to accept 18650 cell, so your rechargeable options are pretty much going to be of following sizes:
- 1 x 17670
- 2 x 16340 (aka. RCR123, R123)

Take note of the quote: "you gain some, you lose some" when choosing an LED drop-in. You can start a new topic in The Bright Side forums at General Flashlight Discussion area to get more input about the various possibilities of "upgrading" your G2.    see you there.