Iain, Yes- you’ll be faster in a group.
You’ll be ready for the distance no problem. Etape Caledonia isn’t super super hilly to be honest ( like the Bealach or Whitton )
IIRC The first section is undulating hills, then it runs flat along the side of loch rannoch for a long while… its this area that you can put the speed in if you sit in with a group … i’d advise that you dont burst a gut trying to keep up before the main climb. Save something for the climb. I’m not the fastest and I tried to keep up with a group on the run up to the start of the big climb and burned out leaving not much energy for the climb.
After the descent, its a little bit more of undulating, then a long flat stretch again back to the start with a climb just at the end. Again, the long flat stretch heading back can be made better and quicker by means of a group.
Group riding wise, if you’ve not done much then there are probably threads on here and other forums about some ettiquettes involved. some road riders/groups are quite formal about it, some less so, but it does help to know the unwritten rules so that you can identify whats happening in the dynamic of the group and go along with it. the better a group works, the faster you go.
For example riding solo, working along I may settle into a speed of about 18 to 19mph … riding the same with a group of about 8 to 10, that speed ups to 21 to 22 or faster if the gropu really wants to up the effort.