Do complex cellular processes (like the cell cycle) have a single rate-limiting step? (I.e., do they follow the Arrhenius equation?) Intuitively, this seems to be the case, as there should be one reaction that is the slowest (and thus rate-limiting).
Now, from the point of fitness, it would make sense to have all reactions occur at the same speed (e.g. by tuning enzymes levels). This would mean that there is not a single rate-limiting step, but rather a series of equally fast steps. Assuming that mutations are more likely to influence protein expression than to increase the catalytic activity of enzymes, this "balanced state" could actually occur.
However, once all reactions are tuned to be equally fast, how do you evolve? If there's only one slow reaction, clearly you have selection pressure on this reaction. If all reactions are equally fast, making one of them even faster (by changing the enzyme's activity) would have no influence, no?
Perhaps a way out is that after the enzyme's catalytic activity has increased, lower levels of this enzyme suffice, thus increasing fitness.