We tested indoors in Detroit, El-Mod and Tamiya could shoot over 100ft easily, the DBC (as supplied by the late Blitz) was good for 60-75ft depending on which emitter was used (3mm vs 5mm). I don't think we did any range tests with the Impact emitter, but they claim 120ft.

We also did some outdoors testing in the UK, but this was on a grey cloudy day. My Tamiya Sherman was good for 115ft, and I think one guy managed to get 130ft out of a DBC - but he'd bought a specific emitter and reduced the series resistor to the point that he'd have to replace it once a year due to burn out. As an aside, it was also quite cold that day, and we noticed that the longer our tanks stood outside, the shorter the range due to battery issues.

When we ran in bright sunlight in Detroit (bright sunlight in the UK never happens ), I recall that it was harder to see the TBU (and especially the Impact battle unit) hit LED's light up than it was to score a hit. I do think that you loose a bit of range/beam width, but I never managed to quantify it.

I wouldn't recommend filing the emitter down - at best you'll be unlikely to get a perfectly smooth (or square) surface and not get any improvement, and at worst it could be considered to be attempting to cheat.