even if apps are 486-compatible, they are optimized for 686 too.
Forgive me, I don't understand what you mean by this. Surely setting ARCH=i686 is more optimised for i686 than setting ARCH=i486? Have i misunderstood something about the way GCC handles ARCH?
In response to your specific questions:
* Is this possible in Slackware?
* At what point is it possible?
It is possible at compile time! Since Slackware packages are binary packages (they dump the precompiled binaries straight into your system) you would have to manually hunt down and compile your own from source. You use disks 3 & 4 for a bit of a short cut in the hunting down of those sources, and these disks also provide the scripts from which the binary packages (on disks 1 and 2) are made.
* Would there be any noticeable benefit to doing this?
Noticable? Maybe. I think in all cases it depends on what the application is. Like the poster above mentions, if you're doing some CPU intensive stuff then you should find transcode (for example) works faster on an i686 with extensions than on a generic i486, simply because of the way GCC interprets this (at least this is my understanding).
* what's the likelihood of permanently pooching one's system by fiddling with such settings?
Slim. At worst you'll set "inefficient" flags - you can't set "wrong" flags, 'cause it probably won't compile if you do! Even if you manage to compile with the wrong flags, it possibly won't run (highly unlikely) thus negating any damaging effects it might have! =)
(For how long I've used Linux, I feel like such a newb...)
As a friend likes to say: Every day's a school day!