Been running 710 bars with 70mm stems for a while and tempted to try out some 750′ bars. The question is will the steering feel sluggish with a 70mm stem? Thinking that at a fraction under 6 ft a 50mm stem may make the cockpit of my medium five a tad short.

Get a 60mm stem then. But wider bars effectively lengthen the cockpit anyway, so if you keep the 70mm you’ll be more stretched out.

yep

So to that effect I am moving from a 90mm stem with 710mm bars to a 60mm stem with 750mm bars
Sound about right?

will the steering feel sluggish with a 70mm stem

the steering wont feel that much different

I’ve gone from 70mm stem with 690mm bars, to 760mm bars (intending to cut them down a touch tho, when I get round to it). Like trailflow, I didn’t feel much difference to the steering, not in the way you’re suggesting anyway, like you do when dramatically lengthening/shortening stems.
Apart from the obvious arms being wider apart, the main difference I found:

slightly improved balance in a straight line and cornering

had to relearn hoofing the bike up steps and large obsticals/step ups, arms being wider apart is a less efficient action, so more effort required pulling up hard on the bars to get the bike up a big step up..

Personally, unless you’re a big bloke with massive shoulders, 690mm-710mm is better IMO.