I don't think this would be good to implement.
With stat increase, what it really comes down to is time.
When players train, they use focus and the corresponding stat. These two refill after 3-4 segments.
Of course, this time can be shortened. Rations immediately refill these meters. VIP and university courses increases the regeneration rate. VIP also "shortens" the time by slightly more than a third, by increasing focus to 150% but not increasing the time it takes to refill. Personal trainer also does the same. Having both personal trainer and VIP simultaneously is multiplicative, so it "shortens" the time by slightly less than 60%.
All of these: rations, VIP, university courses, personal trainers, cost in-game money, which comes from additional time playing. Credits have to be earned for university courses and buying rations off others, university courses take time to complete (and focus aka time training intelligence to decrease university course time). Bonds have to be earned for ration bribes, VIP packs, speeding up university courses by 50%, and personal trainers, but players only earn in-game bonds through careers (only after spending 30 days in the same career and spending the focus to gain experience) and tier increases (which also requires experience, and therefore focus and time).
Increasing the time it takes for training would not solve the underlying issue of players having insanely high stats. In fact, it would further widen the divide between new players coming on or players with low stats and players who do have high stats, since the high stat players would have the advantage of previously training without such a limit. This makes it even more difficult (or rather, it would take more time) to catch up.