An IM swim is in a wetsuit, so it will work out a little differently for everyone. Some pick up more speed than others in a wetsuit.
In my case: 3-4 weeks before IM I did a 10x400 set in the pool (long course meters). Swimming much harder than I did in the IM, I managed roughly 1:45 per 100m pace.
In the IM, I swam 1:00:03, and that was after getting bad leg cramps on the second loop and losing a couple of minutes (lap one was 00:28, lap two was 00:32).
So, I look for 1:45 per 100m in the pool as a benchmark pace for a one hour wetsuit swim.
It is also hard to compare the pool vs. open water. Wetsuit + draft might give you extra time depending on how good you do flip turns, but you also have to swim straight in OW, so you are likely swimming more than 4200 yards. Rough water can also be a challenge - if you do not have an efficient stroke your wetsuit advantage is negated. Craziness at the start of the IM can also slow things down.