Wheel cover and rim question

I noticed a world of difference when I ditched my Alex rims, almost like I was riding a new bike.  Something like a Mavic Aksium or Nuevations would be cheap and well worth it my opinion.

I'm in the same boat, I've got stock Mavics that feel really strong. I want to add a disc cover to the rear.

I've read somewhere that an average wheel + disc cover = respectable aero despite weight of the rim.

Also, what is the agreed amount of wind speed / gusts to say, "not using the disc in this race."

There is none. I've ridden my disc in winds well over 30 mph.

More wind generally means more benefit from the disc.  And cross-winds tend to be more 'problematic' for your front wheel, not your rear.  So pretty much right up to the point that you'd say "not getting on a bike in this wind".