So hypothetically they wont catch up with you unless you drop your own pace to below your normal pace, by which time of course you will lose that very time advantage you set out for in the first place.
To this I have to say "Bollocks" . If your 'pace' is 20kph you will very easily sit in with a group doing 25kph, so they will catch you. But the way things go during the day chances are huge you will not stay with the same group anyhoo. Groups will come and go you could be going with this great paced group and all of a sudden major urge for nature break so you have to stop, bye bye group. When you restart another group will come along. Just roll with it and don't stress, your riding 200k its a long day and there are 15000 odd other cyclists out there, you will pass some, you will be passed by some, you will rest, you will ride alone, you will ride with groups, if you catch the ferry you will probly be cold and crowded and after all that you will most likely have a great day. Enjoy it, don't stress it.
Interestingly enough, heading out to Geelong and then Queenscliff (via Bellarine Peninsula) I never found a group I could tag along with for more than a minute or two. They were all too strong, too fast. I got overtaken by pretty much everyone, except 2 people (husband and wife?), AND all those poor bastards who were fixing punctures. There were so many of them on the Geelong side, and I hardly noticed any on the Franga side.
Now, on the franga side, an urgency took hold of me. We were delayed by ferry issues, and I really wanted to try and finish before 6, then when it became clear I wasn't going to make it for 6 I aimed for before 7
So I took off fast, and I was catching up with other groups. I found this amazing. Either everyone else had tired, or were taking it easy?
I still got overtaken by lots, but I also overtook many people. This astonished me. I would catch up with a group, go with them for a while, then find them too slow and pull ahead of them, hunting my next group. I just ketp doing that, until eventually by About St.Kilda I didn't bother staying with any group, and just powered on to the finish.
The hills still screwed me though. They really take the wind out of my sail. Even so overtook a few people going up the hill too.