Yeh super interesting the list of team mates - Mansell didn't have full seasons against half of the WDC drivers that he team mated with...Mansell had the best teammates in history, we have to give him that.
World champions
Rosberg, Prost, Hill, Hakkinen, Andretti, Piquet.
Prost had only five world champion teammates
Rosberg, Hill, Senna, Lauda, Mansell.
But Prost beat 'em all. Actually he outscored Senna both 88 and 89. 88 points system was just different. Not saying he was better, Senna was definitely faster but Prost was (imho) the best complete F1 driver in modern history. Unbelievably smart tactician, very few mistakes and had the magic ability to make the car last.
That era was so amazing and competitive. 1985 had seven world champions on the grid.
In terms of Prosts outright speed - how much did he adapt his driving to being as fast as he neeeded while being reliable? He had same or more wins than the world champion in 1981, 1982 1983 and 1984 before eventually getting the title in 1985...
And driving with "enough speed as needed" he was able to score more points than Senna in 1988 and 1989 as team mate as you point out.
We would never know if prime early career Prost might have pushed a bit harder and been closer to Senna's pace - but it's pretty clear that in 1988 they had the same car and he was not as fast as Senna.
Mansell compared to his wdc team mates might not be so consistent in comparison of points scored, qualifying or race pace records or by anecdotal comments...