overkill?

Overkill in my opinion.
Until you get a head rechambered and high rpm porting, you are not really hauling any extra air.

Yeah, yeah, I know there are guys who cannot believe this, but your carb is not what is holding you back at the moment.
You can make 30hp on the stock carb, do you feel like you have double the original hp?

Spend you money on a cylinder head and quench gap it.
 
30mm is overkill, your mods will run very nicely on the stock 26mm carb.

Even with a port job and a redesigned head a 28mm will do just fine.

I made the mistake of fitting a larger carb to a mainly stock build, jetting was a nightmare an eventually, when I got the jetting right I gained absolutly nothing.
 
While I agree with and unworked stock motor on a 30mm carb isn't gonna gain much if any on the top end, However, If you plan on having motor work done, It can't hurt to buy a bigger carb now especially if the price is right! Run it and let us know what you think!