The shop I work at charges 96-144 dollars to cut the seats to the size of the valve depending on how difficult it would be to set it up on the machine. That is if the seat has a wide enough diameter to accept the larger valve (which I believe it does) otherwise new seats need to be installed and it would be quite a bit more expensive. plus it would be about 350 bucks to do a complete valve job with tear down, inspection, machining, cleaning, and reassembly unless you were gonna do all that yourself. Ok here is the kicker and your final decision maker. Think of this, If you put in larger valves, the combustion chamber is going to shroud the valve more. So to prevent this you open up the combustion chamber enough to allow for adequate air flow around the valve. In doing so you are lowering the compression ratio of the heads of which the whole reason you have them on there is for the greater compression ratio. So where will your gains be? I dont want to deter you from making horsepower you are looking at this correctly just be sure to consider all the variables. And don't forget the cost of porting. Without going at this with everything considered the only thing youll gain out of larger valves is youll feel really good telling people you have 3L valves in your hybrid.
Oh and by the way I'm shootin for Mani too and I'm rather confident in my ability to build engines. But you must be on the right track seeing as the setup I'm working on is a hybrid as well... and I'm thinking things through the exact same way you are. Good luck with the build
