Is upgrading valves on gt40p worth it? Please school me

Hey guys. I have a few questions that all kind of play into my scenario. First I will let you know what that scenario is. I am building a single HX35 hybrid turbo. I am going to be using my gt40p heads for now, on stock block and stock internals. I am going to be getting new pistons though. So here is the thing, this is going to be a budget build but i want to upgrade the heads a little. I want to replace the whole valvetrain. I dont want to go with huge valves but I want to upgrade them, I think I saw 25th and tmoss say that on gt40p's 1.8X/1.54 valves should be good. My question is to make bigger valves fit what kind of machining is going to need to be done on the heads? Theres no way to make bigger valves fit without machining correct? Anyone have a guesstimate of how much that would cost? I guess thats really all I want to know is how much I am looking to spend on upgrading to bigger valves and if it is worth it. I am not planning on having the heads ported, but I do plan on doing just a little at home cleaning up on them with a dremel.

Oh and one quick question i am having ALOT of trouble getting the valve springs off. I have the heads off the car so dropping the valve isnt a worry. I can compress the spring no problem to give me alot of room to mess with the retainer. But the keepers just wont come out, in my shop manual it says use a magnet and I jsut start laughing real hard because I could pick up the head with a magnet before the keeper comes out. I've tried using an oring remover which looks like a dentist pick, to try to pull the keepers out, doesnt work, I've tried a hammer and a screwdriver on the retainer, I've tried just beating the valve with a hammer, and nothing works, I am just aobut ready to take it to a machine shop and tell them to get these damn springs off, I cant even get one off. Any tips, is there something I am doing wrong, or should they reall just pull out with a magnet....