In terms of weight, I am not sure what the actual numbers are, but the .75" is going to be about 11.6% heavier than the .71".
In terms of strength, I doubt you would be able to tell through casual use, however, that cut out for the 203 on the M4 barrel always looks like a weak spot (probably doesn't make a difference).
If you are looking for an actual midlength gas system, then I would think the closest you could get to this is a RRA SS HBAR 16" w/ midlength port. Armalite also might have this, but I couldn't find it and it may not come in 1/8 or 1/7.
If you plan on just a regular carbine gasport with the mid-ffras, then I think you might be able to get DPMS or Olympic barrels. They would come in the right twists.
However, with all of those, the barrels are usually HBAR or Bull.
You might also try [url=http://www.compasslake.com/Price%20List.htm]Compass Lake Engineering[/url]
As far as I know, Bushmaster only has SS on their 20".
If you can't find exactly what you want, you could always check with Kurt (KKF on industry boards).
HTH