If it helps...
... it seems to be much more important to have a sharp-as-possible chisel-corner on the rim than to have a razor edge all the way round. This is on the cheaper type (wot I have) not the posh Fisch wave bits** with a 'wobbly' rim.
If you think about, it the rim is supposed to excavate a
neat circle, and the rest of the bit 'just' removes the middle. As long as you get that neat circle, the rest is relatively easy for the cutter. It doesn't matter that the rest of the rim (following round) is blunt-ish. Or so it seems.
So on a worn bit (that I thought was past it), I tried squaring-off the leading edge of the rim, and making it into as sharp a chisel point as I could. It worked really well - better than I expected. Then I bought some single replacements from Axminster, and found they were shaped in a similar way.
Above is a really crummy picture*** of what I mean. From the left, those three cutters are:
1. An unmodified original one from a cheap set. It's hard to see, but to the naked eye it looks that the grinder used to relieve the hogging-out part of the cutter is also doing the cutting edge of the rim, and not very well, too. It has a very negative rake and it's blunt (ex-factory).
2. One I fettled. It's rim cutters have been re-shaped to near-zero rake, and to have edges as squared off as possible (i.e. sharp corners, but no burr). It worked pretty well.
3. An unused cutter from Axminster's standard range. It still has slightly negative rake, but the cutting edge is sharply defined and seems to be a separate machining operation from the relief grind for the hogging-out edge.
Both my tuned ones and the Axminster ones cut a lot better than the original cheap set. So it can be done...
The other thing is that, in a power tool of any sort, the "hogging-out" part of the cutter has a difficult job: Imagine that edge is moving in a straight line: one end of it (actually at the centre of the cutter), is hardly moving, whereas the other end (actually at the perimeter) is whizzing along. With circular saw blades we know you can get away with all sorts of profiles as long as there is kinetic energy in the tooth. Mitre saw blades actually have negative rake, for safety. At hand tool speeds, they probably wouldn't cut but instead just scuff along the wood, buit when the blade is doing 3000 RPM, the tooth really smacks into the surface and the cut is started as basically a high-speed impact crossing and shattering the wood fibres*.
It follows then that sharpness near the centre of the 'hogging-out' edges of a Forstner cutter is probably more important than at the outer edge. Again, I've found this to be true in practice, so I now pay more attention to the inner bit of the cutting edge, and really only de-burr the outer edge. But the outer edge does wear faster, so it seems it gets more attention than it really does.
Hope that helps. I use diamond needle files, because (a) they are cheap, and (b) they cut faster than traditional needle files, and (c) they last well, so I'm not picking up blunt ones all the time! But they are not precision tools - there's usually an errant larger fragment of diamond embedded somewhere. the trick is to find it and work around it.
E.
*I'm not citing any sources for this, it's merely my hypothesis. Neg-rake blades aren't used for rip cutting, as they probbly wouldn't work at all well, irrespective of the sides of the gullets.
**Those "wave" profiles probably work so well because they act like tiny circular saws, severing fibres neatly as they pass. The problem is that most Forstners I've seen are relieved around the rim. I mean that the outside is very slightly conical. So you can resharpen the traditional type if you are maintaining the cutting edge as I've described above, and that shouldn't change the diameter, But you can't resharpen the wave edge because as you wear it down the diameter reduces, albeit ever so slightly. It probably makes no difference in practice, but it's a similar problem to honing router cutters - don't do the edges of the tungsten inserts!
***My tablet camera (only thing handy at the breakfast table!) seems to have a firmware bug: if you manually zoom in for macro stuff, the focus seems to shift between preview and the actual image. That image looked to be quite a bit sharper than it turned out! But photographing shiny things is always tricky.
