The rosters would have been completely different. It is impossible to determine what the teams would have looked like those years- would they have looked more like the seemingly-3-losses-every-year teams of the 80s? Would Tress have made that recruiting jump on his own?
To some extent every coach builds the base a little higher for the next- Cooper's escalation of recruiting set a higher base for Tress to start from, Tessel's efforts built that base up for Meyer, Meyer's work built it for Day.
The question you are asking has two components- 1) What would Tressel have done with Ohio State recruiting starting with the base in place in 1998? and 2) Would Tressel have coached similar talent, if it existed, to better conclusions than Cooper?
I'll say this- give Tressel the rosters Cooper had and I think he wins one in the 90s. Winning a title takes a lot of luck and good breaks. (See: Recent ass whipping of Clemson resulting in a loss as a bad example, Holy Buckeye as a good one) so I would just never predict more than one.
Now, ask Tressel to start from 1988? Well, there I'm not so sure Ohio State would have been as competitive through the 90s.