MkVIII,
While I have not flown the B727 (and am certainly not an aerodynamics expert...just a pilot), the older models of the 707 were certainly a challange with regard to runway performance.
These early aircraft (straight pipe powered -300 series, with minimal leading edge devices) would roll...and roll....and roll, seeminging right to the very far end of the runway, before rotation was possible, at heavy weights.
A large part of the problem was the non-fan engines, but also the lack of full-span leading edge devices.
Only one airline opted for full-span leading edge devices on early -300 series aircraft...South Africian.
I flew one of these for another operator after it it was sold, and the runway performance was greatly inproved.
Along came the fan engines, and full span leading edge devices were fitted as standard, on the long body aircraft.
The only aircraft that I have flown with slats was the Lockheed TriStar, so I really can't compare with others.