A few things to consider.
You can't just do a drop in LED upgrade on most (almost all) current Surefire LED lights. If you want a newly produced Surefire host for P60/D26 modules you'll need to buy a classic incandescent 6P or G2. The other problem with the 6PX/G2X/Fury Pro models is that they alternate between high and low with each button press.
The Solarforce L2P is an excellent host for P60/D26 LED modules. It's type 3 hard anodized, and has a larger battery tube internal diameter than Surefire lights do, so you can use 18mm diameter rechargeable lithium batteries.
I wouldn't buy a Solarforce drop in for any serious use, much less weapon mounted use. The Solarforce drop-ins just aren't assembled properly for hard use. If you're okay with Chinese components that are better assembled buy a drop in from Custom Lites. If you want the most rugged drop-in made get a Malkoff M60 or M61: You'll give up some output, but you'll gain run time and get extreme reliability. When you look at Malkoff's output numbers also keep in mind that he gives you the lumens as measured coming out of the light, while almost every other manufacturer gives you the lumens at the emitter which inflates their numbers.
You also need to look at the LED used. A Cree XP-G2 produces less lumens than a Cree XM-L2 because the G has a physically smaller emitter. However, in identical sized reflectors the G is more intense. That is, the light output of the G is tighter, so it will illuminate objects further away.
Other lights to look at:
Malkoff MD2 with M60 or M61 drop in of your choice. The high / low ring option works by fully tightening and slightly loosening the head so you can control what mode it's in. Also has 18mm battery tube.
Elzetta with M60 or M61 drop in of your choice. High/low capability is available through an optional full tight / slightly loosen tailcap switch assembly.
ArmyTek Predator Pro (XP-G2) or Viking Pro (XM-L2). These are max output with the head fully tightened, and user programmable lower output with the head slightly loosened. Battery tubes are 18mm compatible from the factory. The electronics driver is optimized for both 2xCR123 primary or 1x18650 rechargeable. The assembly and build are also very rugged. These are made in Canada from US and Japanese components. The engineering is done by the owner, a Russian expat. She also works the booth just like knife company owners at shows like Blade. I own an early version of the Predator and it's a GREAT light.