After USPS lost my almost new LM 21W LED in the mail when it was on the way back for warranty repair (sigh..), I think I'm in the market for a new primary light!
Sorry to hear that. I went through a similar process of moving away from corded to cordless primary lights. First, for run times in the 1 hour range in overhead environments, then in the 2 hour range. I do typically two dives, so 4 hour burn time (full power) is what I was looking for. I considered the RX8 seriously (it was on sale for $275 at DGX at one point) but decided against it. Reviews are spotty but they indicated rapid decrease in brightness (after about 30 minutes) and it's bulky. The LX20 is nice but expensive at $699. If they sold it w/o QRM for ~$500 I might have gotten it. I tried it out (out of the water) but wasn't too impressed. Maybe it was the 6 degree spot (I prefer 10 deg or thereabouts) but brightness seemed so-so.
What I have for cordless primary:
FinnSub 1400 Short (stay away), TillyTec 1500 LED (great light); both are bulky (like the RX8) but ~4 hour run time at full power. Drawback: both were state-of-the-art about 2-3 years ago and still remain expensive.
Bigblue TL2500p; super bright (I'd say about as bright/brighter than typical 21W corded LED primaries) and it uses a single 32650 battery that can be changed out between dives. I have a second light head, VTL2500p, which you can operate as a video light and a 800 lumen spot light. The 2015 model is the TL2800p (just 200-300 lumens brighter they claim). Cost is $375 and for your run time I would recommend giving the TL2800p some consideration.
Bigblue TL4000p; with ~2 hour run times, the TL2500p doesn't cut it but TL4000p does. It's not cheap (around ~$550) but ~6 hours run time at 2000 lumens (around ~3 hours at 4000 lumens). IMO, it's overall the best cordless technical light currently out there. In my experience, some of the run times could be a bit exaggerated but even if it's conservatively ~4 hours at 2000 lumens it's crazy bright. It's bulky (battery pack of 4 18650 batteries) but unlike the TT 1500 you can change out the battery. The LX20 is less bulky (even though it uses 4 18650 as well) so for that reason alone I would have gotten it had they sold the light for ~$500. I use QRMs in my helmet and goodman handle, so don't need a light that comes with it. Being compact (which the LX20 is) is a plus when helmet mounting it. Hope this helps.
BTW, if you're not going into caves (doing wreck dives with actual overhead bottom time ~30 minutes or so), using backup LEDs such as DRIS Shorty, DGX 600, BX-1 (and others) as primaries will do. They are plenty bright and will run for close to an hour at full power (on good 18650 batteries). I helmet mount the DGX 600 and BX-1, and keep the DRIS Shorty (uses AA batteries) in the pocket just in case.
