GlueGuy is right, solar modules generally max out at about 80% of rated power. But the power derate comes as reduced voltage as the modules heat up in the sun, the current stays about the same.
That's why modules designed for 12V battery charging are around 17V max power, way higher than you ever want or need for a 12V battery. Are the solar manufacturers cheating you by rating their products this way? Not intentionally, its just that when the flash test them at the factory they do it at room temperature because there's not time to let them heat up to a more normal operating temp.
So if you keep it clean and aim it at the sun you can and probably often will get that 4.6A out of your 80W module. Two of those will be over 9A which is pushing a 10A rated controller. So I'd suggest just leaving your existing Zamp kit as is and getting another 80W module and controller to add on.
You can get a 20A PWM charge controller and 80W module combo for around $100-$120 on Amazon. IMHO Zamp's prices are crazy. Or get another size module/controller if you want. It doesn't matter which brand or size of module. As long as its max power voltage rating is about the same as your Zamp one you can operate them in parallel no problem. Read the comments and reviews of course so you can avoid the junk...