I think the P61 was rated to 120 lumen (actual rating was probably more) for ~20 min. The Malkoff M61W (The W at the end means warm white, more incan like) does 200 lumen for 2 hours. It is single mode and costs $60, but it is well built and well recommended. The M91 takes up to 12v, and the warm white version give 500 lumen out the front and should run for ~1.5 hours.
To get more output, a larger emitter is needed, and that means a wider beam due to the larger emitter size. Malkoff also has lower output modules for long runtime.
A single 17670 Li-ion battery should fit the 6P from what I've read (I don't own any surefires). It's 3.7v (4.2v fully charged) so you'll have to make sure the drop-in accepts that voltage. It's around the size of 2 CR123 batteries and slightly wider. 2 of them in series can replace 4 CR123 batteries.
There are also other drop-ins, one of the brightest being the Torchlab triple with 1600 lumen stated, and it is available with different modes (does cost $165, and is currently sold out at Overready). Also check the Custom Buy/Sell/Trade forum for higher price really bright drop-ins.
There are also many cheaper drop-ins (under $30) made by various companies such as Thrunite, Solarfoce, and some don't seem to be linked to any company.