I've been running Goodman Games's 5E conversion of Into the Borderlands (Caverns of Quasqueton + Keep on the Borderlands). Probably going to need to pick up the Isle of Dread soon as the party's 3rd level and oversized (7 PCs).
How is running with 7 PCs?
Not as bad as you'd expect.
The game started out as a one shot to test drive 5E, with three players. We realized the party needed to be fleshed out a bit, so each player opted to run two PCs for a full party of six.
We added two other players a little later. One took up an existing character while another opted for his own, giving a total of five players and 7 characters.
Biggest issue for me is how seven PCs will absolutely maul some encounters. I'm going to dial up the heat a bit and see how they fare, give them a toughened encounter or two.
It helps that most of us are old friends and we all know each other's foibles
Having run for groups that typically have 7-9 players, and at particular sessions, more than that, I'll suggest for 5E the easiest way to run for the 7-9 group is to make double-sized encounters and completely ignore the 5E advice on scaling. Assuming you are playing fair on the "focus fire" aspect. 16+ goblins shouldn't all be so disciplined as to peg a single character, for example. In fact, the way I think of it is of two smaller parties that happen to be adventuring together and
two groups of foes that happened to show up at the same time. The benefit that a party gets out of combined actions is greater than what even diverse but cohesive monsters get. In other words, 7 characters scales to more power compared to 4 characters than double monsters does.
This does all kinds of nice things for your ability to adjust on the fly based on the actions of the characters and a varying roster of players showing up. When the party size drops temporarily into the 4-6 range, they've always got the option to try to deal with the encounters piecemeal. And they've probably got a better chance at sneaking around to make that happen. It also gives you a lot more leeway at shaping encounters to the environment instead of doing that "balanced encounter" dance. There are, say, 8 groups of goblins sitting or moving around in the vicinity. Depending on what the players do, they could handle 1 or 2 of them isolated and then sneak around the rest. Or they could call all 8 groups down on their heads in waves and probably die. As it should be.
But mainly what it does is let you build things in monster groups that would be roughly appropriate for a typical size party and your overall goal (cakewalk, can handle it if not stupid, probably should run, etc.)--and then ignore that aspect of the game thereafter.