I don't usually have a road to drive that fast for any specific length of long time - so I can't tell you of the long-term effects or results because by the time you get to open roads around here - there's is the usual results of TXT-ing while driving idiots that start to plug up the lanes as they settle in for a longer drive commute and their Collision-avoidance system then backs down to about 80MPH in most vehicles.
- Why that is? Reasoning varies but usually it's the smoothness of the road - cracks in the pavement and other factors inclusive (Semis Busses - larger transports)
- Ever try to type on a bus? Those shifts in pavement usually force you to brace against something and by doing so - tires you out faster than walking that same distance.
- Other factors also include the ability of the Drivers to see...beyond...(I'll just leave it at that)
But to me the "buzz" the motor becomes to my ears turns into a painful ringing distortion - usually about 4K...so since the gearing between Auto and Manual "overdrive" ratios are different - the Auto side is usually quieter at higher speeds due to the "longer gear" ratios Autos have in Overdrive.
But you asked about "stress" - once on the road your main "drag" is the wind and terrain - the type of road considered smooth - you'd not have "stress" perse' - but torque to momentum acceleration starts to drop after the engines main torque band is exceeded - again, usually about 4.5K in RPM - once your there in top gear - you're already going way too fast to control the car in a usual sense. Then the stress issues then become more of control and steering.
So were you figuring roll over or braking or just torque to speed changes?
If it's acceleration, then the ball usually drops in N/A vehicles at about 65 to 70 MPH (in 5th Gear manuals that is going above 3.5K) but doesn't answer the question of which Gear you were thinking of using to attain that speed. Because, in say - 4th Gear, the upper limit of torque present to gain speed is different - speed is less but the torque to provide acceleration to raise the speed to the middle of 5th's working torque band is greater in range - ranges up about 4.5K)
ST models having a turbo boost gives you a longer - range - of RPM as a torque "band" to work with - can get you the 90MPH value easier and more quickly but still have limits on the torque band range versus throwing the turbos bearings out thru the tailpipe from the keep pushing the limit - figure of merit - approach. The - "Labor too hard" you mention.
So there are several ways to look at "stress" on the Fiesta - one of control, another of power - Torque considers Acceleration, even the mechanical and then there's the abilities of the driver
Because stress-factors on the car in lower gears, to the Frame, can be harder on the Engine mounts than say the car having already attained a cruising speed, it's harder to pinpoint a "value" to give you - so are you looking into something to work with, around RPM, or for power to weight ratios in Speed?