I just did a DIY upgrade to LiFePO4 batteries and it's going well. I used twelve A123 20Ah LiFePO4 cells and I just did a full burn test that gave me 2h40m of trigger time on second speed, pitch 7, in a sidemount configuration with dry suit. The costs were less than $1,000 to upgrade the scooter. The original NiMH battery pack was rated at 480 Wh of energy, the new LiFePO4 is 800 Wh, almost double.
Because the new batteries have a different voltage I needed a new speed controller. I probably could have bought a Venom controller from Silent Submersion but I decided to create my own custom speed controller to reduce the 40V of the new battery down to the 18V, 24V, and 30V used by each speed. I took the opportunity to program the new controller to start in 2nd speed, to beep to indicated which speed was in and to beep to indicate a 20 minute battery warning before limp mode.
I'd be happy to provide the schematic, source code and PCB layout for the controller. The components can be purchased for about $50.
There are some small issues.
The H160 already had poor trim in fresh water and this battery made it worse. I added 2.5 lbs of lead to the nose and added two 2"x10" capped ABS pipe to the tail to compensate, it's now back to normally bad trim. On the positive side the 2.5 lbs, placed offset, reduces torque force on the handle quite a bit.
There is a slight wobble in the motor that I have to try and work out.
I didn't hook up the LED light since I don't use it.
The original battery pack was waterproof, this one would be exposed to water if it were to enter the hull.