"Do you ever worry that all of these amazing parts could out-fabulous the frame? Not to suggest the frame isn't fabulous, because it is."
In a word, no.
Trek made some nicer steel bikes than the 85 720/620 here and there, but the 85 720/620 are really both icons of the Trek steel years and the tail of the bike boom, the post-hippie/post-bikecentennial era when "touring" was something everyone wanted to claim about their bikes.
Among touring folks who aren't particularly vintage or Trek inclined, they are also a pretty iconic model.
So w/all that aura, again, no. If it were something else, like a Trek 660 or something, fine bike but w/out the buzz, yeah, I'd be concerned.
I think the way I'm going about it reflects the vibe of the bike - I'm not going necessarily for the raciest, lightest or latest stuff, for the most part. I'm looking for components that are top quality in a balance of performance, durability (emphasis on this), and somewhat neo-classic timeless styling. Sort of an ultimate ride it to wherever touring bike.
Lots of people will still say to this day that the 720 is/was among or the finest touring bike. This is arguable, for sure, but there are definitely people that feel this. The 620 to me is even a little better since it is made from slightly stouter tubes and I'm not a lightweight. Plus, for a long time, since the 620 that was a fraternal twin of the 720 only came out for one year, it was not nearly as well known as the 720, had kind of a cult/insider thing about it.
So again, no, I'm quite comfortable with this. My previous 620, which was ridden very hard by me for 10 years, had a lot of high end, durable stuff on it - I loved the Dura Ace RD-7700 GS triple rear, but you can't find those NIB now w/out paying an arm and a leg.
I never felt then that I was outclassing the frame. I'm ratcheting it up a bit now, but some of that is just mad money from the great vintage parts sale. And the funny thing is that the prices I'm paying for some of the new stuff isn't all that extreme when I consider the prices I'm getting for some of the vintage stuff. Particularly when you consider that the vintage stuff often is used & depreciated for that, etc.
I think that the Royce Titan Venus hubs are the only thing I'm buying that is more than 2x the price I got for a vintage comparable component. FWIW, price of some of the stuff on the agenda for this bike is less than I've sold vintage components for. And had I gone w/Shimano Dura Ace or White Industries H2/H3, I still would have been w/in that 2x range, especially when you factor in what I've gotten for a freewheel ($100) vs. what I'll pay for a cassette ($60 or so).
Of course, it is cricket to put anything on Reynolds 531 frame, no part is too good for this, according to the rule book.
Finally, unlike a lot of other vintage bikes, the mid-80's tourer's are not obsolete to their intended purposes, or at least the case is more arguable than for race or mtb. I think it would be a little ridiculous to be putting carbon cranksets on an early 80's Pinarello or some crazy modern/expensive stuff on the Bianchi Grizzly I'm dabbling with.
However, people are still making/selling frames much like the 620 for touring/allrounder stuff. By that standard, the only thing "obsolete" about this bike is the threaded 1" steerer and the canti-post positions. Part of this project is I'm going for what I think is the best of vintage and modern. I still believe that vintage frames are a bargain - get something like this 620 new is 1500/2k or so.