So now that I torqued them to 20+, should I be concerned about them being stretched or otherwise stressed, or just leave them be? :-/
Well, and you probably aren't going to want to hear this. Given that whatever torque value you have imparted to the bolts, they are equal and didn't break int he process, I would leave them the hell alone now as to do anything else invites their total replacement.
What I would do is have a long hard look at your torque wrench. If it was expensive and looks to have adjustment points, I'd be taking it to a good tool shop to have it correctly adjusted (or do it myself if I could figure out how).
If it was a cheap one, or is non-adjustable it correct placement is in the nearest bin, or sent anonymously to a BMW owner you do not like.
It is reading way under correct spec, an M6 bolt is broken long before it gets to 20ft/lbs, an M8 would be marginal.
In fact if I recall correctly the general engineering recommendation (and I am at work and therefore operating on memory) is around 7.5ft/lbs for an M6 and 11ft/lbs for an M8, assuming both are 1mm thread pitch, general grade bright mild. You could add 50% more for high tensile steel and double for titanium.
Now there is a thought, a long time ago a dumb mate of mine who worked in aerospace thought it would be really cool to replace all the M4 and M6 Cap screws in in his Yamaha engine covers with titanium cap screws purloined from work. Shame he didn't use any thread lubricant and an even bigger shame that was about 2 years before he had to try to remove any of them.
Given that this was an XS750 and at the time replacement engines were neither hard to find or expensive, he gave the old girl a birthday with a later model XS850 engine as that seemed cheaper than paying to have the 4 bolts he broke trying to undo and their remaining 20 or so cousins electrically eroded in an attempt to save his existing engine covers.
