Interesting.I had a manual 91 5sp model, admittedly it had a trick or two added but these things 30 years ago were still pretty punchy..and only weighed 1300kgs.
I also dont recall it getting tired at the speeds you mention at all.
Issues imo were(and mine was still humming no shit)were the 180kmh speed limiter and probably the handling which wasn't great.
Where your probably right is it had a small sump which was a real issue as I lunched my motor
I didn't change the oil and filter regularly enough and tbh i gave it a lot of shit..often lol
Good car though for the era, it smoked VL's and was quicker than the 5L too.A.mate at that time had a comparable XF falcon 4.1(auto) and it couldn't get close.