I have a Terralux TLE-300M-EX in my 4 D Cell Maglite and it's great. I have some information in these threads:
https://www.candlepowerforums.com/threads/258079
https://www.candlepowerforums.com/posts/3182367
Before I bought mine, I read this thread on the predecessor, the Terralux TLE-300M:
https://www.candlepowerforums.com/threads/201059
That earlier model is similar but it doesn't put out as much light and it only has a high mode.
The reports there about the beam pattern apply to the -EX model too and my views are the same. The TLE-300M-EX does not have any spill. It just has a big, wide hot spot that is in a star shaped pattern. The hot spot is not round like what my TK40 produces. When you go "white wall hunting" with it, it is not a pretty beam pattern, but in actual use outside it doesn't really matter to me. If I want a drop dead gorgeous beam pattern, I reach for my TK40 and nothing else so the TLE-300M-EX's weird star shaped beam pattern doesn't bother me at all. It does get rid of the much despised (to me anyway) donut hole pattern of the stock Maglite. The beam pattern is devoid of rings, dark spots, and any other artifacts. It's best described by other CPF members as a "wall of light."
The TLE-300M-EX is very easy to install. The included instructions are easy to follow and if you can change the stock bulb in your Maglite, you can install the TLE-300M-EX. I added a layer of Arctic Silver 5 thermal compound to mine as I described in the above threads and it really helps dissipate the heat to the aluminum Maglite body. From what I've read, heat dissipation is one of the problems with this drop in upgrade. I never tried using mine without the Arctic Silver 5, but with it mine doesn't decrease in light output from overheating at all, even with extended use.