On pg 171 of the BRB for Scout: "...a unit containing at least one model with this special rule can redeploy." Since I'd be sticking the Boss with the Koptas at deployment, they would confer Scout to him, if I am reading it correctly. This gives him a nice 12" move right out the gate, with the Koptas to take wounds for him while everything else catches up. A Painboy on a bike would give them some extra survivability while they get into position.
I can see why you'd want Grotsnik, to have a big blob of 30 Fearless Shoota Boyz, but isn't it a bit excessive (at 160 pts) just to have that? The only other reason I'd take him is to act as a bullet sponge with a 4+ save, and at that point a Big Mek would be cheaper (although wouldn't give Fearless/FnP). Although I wouldn't have the durability of Grotsnik, his T5 would largely go to waste, and his incredible number of Power Klaw attacks wouldn't see much use while I'm actually shooting with the Boyz unless I threw them into combat (kind of defeating the point of having Shootas
My hope is that the Boss/Koptas + Rokkitwagon would be scary enough that my opponent would have to choose one of those rather than worry about footslogging boyz firing potshots.
Additionally, I wouldn't be able to afford him without dropping the Boss + Painboy entirely, or including him in a higher point game. If I took this list to 1750 I could surely drop the Painboy for him + some other goodies.
On the topic of Painboys: Is it worth dropping the Buzzsaw from the Koptas for the bike? Since the point of the Koptas is bullet sponge with some slight anti-armor use. Since you make that point, I want to say yes, but the utility of the Buzzsaw on the Koptas makes sense to me.
Mek with a Rokkit would make sense, but I assume that when my opponent sees that Battlewagon cruising at him that he'll use a lot of his shooting just to try and cripple it so he wouldn't get to use it that often. And I'm not sure where to pull the 5 pts from except from Bomb Squigs (which are far more reliable). At that point I could drop the Mek + 2 Squigs for the Painboys bike, but again I'm losing utility for that.