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FWIW, I've found the LED retofits for incandescent auto tail/stop/marker/turn lamps to be marginal at best, and downright mediocre compared to standard incandecent bulbs as far as light output/lumens and the relector "fill".
As far as I can tell, some (often cheaper) LED modules are junk (i.e. the maker compromised on light level and/or LED beam spread, to make them initially cheaper), and this little fact has given the entire LED (car light) market a black eye.
My personal experience is that you should shop for LED modules based upon specs (not just the car socket it fits, but also: light output, light color, and beam angle) for your application, and NOT on price alone (as the market for LED modules is even more variable, in terms of what you are buying, than the market for normal bulbs). But my experience is also that the "top end" of the market in terms of LED module specs (which may only be around the middle of the market, in terms of cost, so shop around), will usually greatly EXCEED the "useful light" (in this case, the portion of the light that hits and goes out the car lens) of a standard (incandescent) car bulb in the same situation!
And remember, when comparing total lumens (of a LED module vs an incandescent car bulb), the LED module will often have two key advantages vs the standard bulb (both of which can raise the "useful light", relative to a standard car bulb, higher than it at first appears):
1) LEDs can be designed to focus the beam angle at any angular width the LED maker chose. The down side of this, is that LEDs that are focused too narrowly, result in the problem of the light not filling the car lens, that you previously mentioned (was your experience). However, the advantage of this, is that if you properly match the beam angle to the usage in your car, you will put more of your total light out the lens of the car (as you are pointing the light right at the car lens), and have much less of the light wasted (going out the sides/back of the bulb, only to have it partially reflected back, for example).
and 2) Normal incandescent bulbs are naturally white. So to get colored light out of incandescents, you start with white light, then send the light through a color filter (either a color filter/coating on the bulb itself, or more commonly a colored car lens to do the filtering). This color filtering works, but wastes a LOT of light (in the case of red tailights, you can easily waste about 70% of your car's while light, just to change the color to red). OTOH LEDs can produce colored light without the filters. So if you match the LED module to the lens color, you avoid the lost/wasted light, and instead have almost all of the LED light go through the car's lens. This fact alone can considerably raise the effective light of a LED module vs a normal car bulb.
For example (to put real world numbers to this), an approximately 180 lumen red LED brake/tail-light assembly (and yes they are out there, I was able to find/get 180+ lumen red LED 1157 bulb replacement modules, for around $25/module) will be about 1/3 brighter than an approximately 400 lumen stock white bulb, counting only the white/incandescent color filter losses (point #2, above) alone (as the 400 lumen bulb will be reduced to around 135 lumens of useful light (give or take, due to the exact car lens design), due to the car's red lens alone). If you also properly match the beam angle of your LEDs to your usage (so that you fill the car's lens, but don't put much light where it's not doing you any good), you will also get more of your total light out the lens (point #1, above), vs the stock (incandescent) bulb.
The final result (between both of these benefits), is that (in some cases, it depends upon the exact bulb application) even LED modules that nominally have fewer lumens, can end up with more "useful light" (i.e. light through the car lens) than the (supposedly higher lumen) stock incandescent bulb. In my case, those 1157 bulb replacements (I previously mentioned) appear (to my eye) to be about twice as bright (in terms of brake/tail light out the car lens) than the stock 1157 bulbs they replaced. But this isn't all that surprising, when you run the numbers, since just the lens losses themselves should make the LED assembly about 1/3 brighter, and there are also "beam angle" benefits by using the 135 degree LED module vs an "all directions" 1157 car bulb!
Bottom line:
LED retrofits work well, if you get the right LED retrofits. But you have to be careful which ones you buy, because you are correct that many LED retrofits (especially the cheaper modules) just don't work as well as stock. So it's "buyer beware" when shopping for LED retrofits.
Someone at my job (not me) had gotten stopped for no tag light, and though the lights on the tag of our car are two years old almost, I decided why not try them out?
