I don’t know about the original H51w, but I can compare my final rev H51w to the new H52w.
The H51w has a smaller emitter and a more focused hotspot, so I would expect it to throw better at similar lumen ratings. The H52w has a much more efficient driver though, and can get both more lumens and longer runtimes even if the hotspot doesn’t look as bright due to being more floody.
Here’s a shot of the H51w at 0.18lm (left) vs the H52w at 0.34lm (right). To my eye, the H51 has a smaller, slightly brighter spot, and the H52 has a larger spot. I can’t easily compare the spill brightness right now because it’s daytime and I need a bigger room. Either one works nicely as a night light, bounced off the ceiling in my bedroom — bright enough to see the whole room clearly but not enough to keep me awake.
Both have a brighter spot than the L3 L10-219, which is spec’d at 0.09lm. The L10 is actually a more appropriate level for my 4am trips to the restroom though; 0.34lm on the H52w is bright enough to be a little uncomfortable… and the 0.06lm setting on the H52w seems just a touch too dim (but is still quite usable).
Results are similar at the next setting up (H51w at 2.2lm vs H52w at 2.7lm). The H51 seems to have a smaller, slightly brighter spot which is brightest at the center, and the H52 has a larger, slightly dimmer spot which appears subtly donut-shaped. Also, I couldn’t get my camera to show it, but… in person the H51w looks sickly yellow to me while the H52w looks much closer to white:
In any case, pretty much every level shows a similar result — H51w has less overall output but a smaller/brighter yellow spot, while the H52w has a larger, whiter spot. The H52w has a much higher maximum output though, so its brightest setting should still throw farther than the H51w.
I’ve found that the H51’s narrow spot is nice for looking at things far away while biking, but the H52’s wider spot works better for almost everything else. I tend to bring my XinTD C8 while biking though, so I don’t exactly need much throw in a headlamp.