The kit you posted is for the G&G L85. I don't know if the cylinder head and nozzle are the same, but the sector gear is different. It has two tappet plate posts for the daffy precocking thing the l85 does. I'm not sure exactly what that would do to an SR25, but I'm sure it wouldn't be good.
For the barrel, I would stick with something that isn't 650mm and madbull. Maybe a nice 510mm 6.05 prometheus? Also, don't forget to mess with the rest of your hop-up as well. A&K chambers aren't the best. I would go with a pro-win or madbull ultimate with a marui bucking personally. At the very least get a new bucking with the barrel.
One of the guys on my team runs a CA AR10 and he uses the next highest spring. I'm pretty sure a PSG-1 specific spring would work as well, but you don't pick the next highest for that.
Also, you can probably find a better ACM motor for much less than the G&P. I haven't looked into motors in a while so I'm not sure if theres anything new/good out, someone will probably throw a few brands at you in a bit though.