Boring dosent give you any more power.
If your talking about going to a big bore kit then yes it will give you more power. Boring is used whenever the cylinder has enough wear to be out of tolerance with the piston and rings, therefore losing compression. So you bore the cylinder to the next size. Boring the cylinder over to the maximum it can handle will probably only give you .5 hp
If you want more power out of the thing look and gettin a higher compression piston. wiseco makes like a 10,11, 12:1.
You can also look into getting a camshaft to change your power range. Dont get a high rev box you can rev your quad to 15,000 rpm but if im not mistake a 300ex stock cam makes most of its peak power by 5-7k rpm. so getting a rev box to let you rev till 10k rpm dosent do you any good either unless you have a camshaft that allows the engine to make power untill 10k rpm.
Realistically Id sell your 300ex and buy a 400ex. The cost difference would be minimal and you would be starting out with about 10 more hp.