"The point of diminishing returns occurs as soon as you buy a stock Kilo TT. No amount of upgrading will make it much better."

So then, the stock frame is relatively poor? The faults I've read are relatively high weight and flex, but this last doesn't seem to be a common criticism. Does this jive with your experience or are there other issues?

" The point made about the fat tires is well taken...buy the bike that has the best geometry for your needs and forget about upgrading. Replace or add parts according to your needs, not according to your desires."

That's a good insight.

Perhaps my OP was unclear, but I'm not terribly fussed by tire width. The big blocks clearances are an asset, but they aren't a "must have".

I suppose I should have just asked what can I, as a new fg cyclist, expect to see (performance wise) in any the other bikes that I won't see in a kilo.

I typically ride 80-120 mi/week. Indoors when it's nasty and mild rolling hills when it's decent. I ride a 20 year old beat to hell road bike, which I believe is stock aside from lube and tres