Actually yes.
A few in fact.
If you use the cb650 head, you will need to use a cb650 exhaust as well.
Unless you want to choke the new head you just put on.
Another is if you go with the cb650 head you will want to use cb650 pistons.
With the 60mm GPZ600 pistons and the 650 head will result in lower than stock compression by a bit. (mabey 8:1 or so... good turbo pistons with a 650 head?? isn't 9:1 stock?)
But they could be checked and the CR calculated I guess.
It all depends on what you want to spend your money on.
Porting the 550 head will probably gain as much as the stock 650 head will give you i'm guessing.
Running with the 650 head means you don't have to port, but you need pistons and exhaust.
If going on a budget....
I'd do just the 60mm pistons and a cam or something. Probably carbs as well.
You already got the cam, carbs can be had for under 100 on fee bay, cb750 carbs are easily cleaned and in running order for cheap. Even with rebuild kits.
I bought my set and got rebuild kits for about $80.
And you already know the price for the 60mm pistons... a song pretty much
If I had the cash...
I'd go with a turbo, port job on the 650 head, 650 cam(perhaps the 550 cam would be better for turbo?? or a reground 550 cam with a turbo profile), as well as the GPZ pistons.
Turbo would need custom exhaust, a big carb, and intake plenum.
Just checked, yup, megacycle cam #126-x1 is a turbo profile for the SOHC 500/550's.
$$$$ for this setup i'm thinking.
l8r