I'm pretty certain, as "lasttoleave" stated, that it's not the bit.
There are a lot of variables, not the least are the router and collet. There's also how securely the router is located relative to the work piece and how planer the work piece is as presented to the router base.
The Festool 20mm boring bit, 491072, has a very sharp perimeter cutting edge that normally maintains a near perfect hole surface but if it isn't plunged straight down, it will open up the hole. The LR 32 set up works so well because it locates the router quite securely in the intended spot and a clean plunge is easy.
The 8mm collet is brand new as this is the first 8mm bit I have owned. But in order to verify this I have actually done some more testing:
Firstly, I removed the larger triton from the table and put the 20mm bit in the collet. I then inverted the router and protruded the bit just high enough to expose the visible shank. I got a dial caliper clamped to the router base and measured for runout. it's within 1 thou, so I am now certain it is not the runout of the actual collet/arbor.
I bored a few test holes with the larger router in the same manor, I screwed it to an oversized base plate and tightly clamped the base plate to the MDF, which was clamped to my MFT. I can't really see anything that about this set up that would yield such different results than others.
I have tried varying the motor speed and plunging speed with no real difference. The bit just seems to want to burn a lot, so I have to raise it back up to remove some loose chips and to reduce the heat before plunging back down, the hole clears in about 2-3 plunges, but it's a nasty result.
I should also mention that without the height lock on (for obvious reasons when plunging) I did notice a tiny amount of play in the smaller router, which I assumed may have been the cause, but the larger table mounted router has absolutely no detectable play in it whatsoever, probably because it has lived in the table since day one.
Should the bit be expected to cause burning quite this badly? I am running out of ideas on how to remove any more variables. I am running on the slowest speed but I can't work out why the holes aren't coming out clean.