in my time in ironman i have improved from 10:18 on a 1:03 swim to 9:02 from a 50 swim.
i have found that the reality of racing and drafting works like this.
if you swim around an hour or an hour and change you and up in the front of the middle of the pack. consequently, you are surrounded by reasonable cyclists who intentionally or unintentionally from large bunches that are next to impossible to police. also, importantly, they are almost next to impossible to escape from. unless you can ride a 4:40, you end up on the front of a bunch, towing them along. so, most people decide to spare their legs and roll around with the bunch rather than ride for them. as such, you can only ride at the speed of the bunch that you end up with.
the reality of the situation is similar, but not as congested, further up the tree. at a 50 minute swim, you end up with the pro field, and if you have reasonable bike legs, you can work from the pace (and not draft) off these better athletes.
so, my theory is that regardless of your bike strength, you usually end up with a bike split decided by your swim, unless of course the race is full of hills. the more turnarounds and the flatter the course, the more important your swim becomes.