You asked for input, so here is my opinion.
I have not known any quality engine builder/porting guy, locally or around the country, that has ever wanted to do a 5 angle job on the turbo Buick head unless it is for only all-out racing, and NOT for street use. A 5 angle job will end up melding into a 3 angle seat after a few WOT blasts, and actually will gain you almost nothing on the street except shorter life of the valve job.
In a race situation heads are removed often, so they have a "touch-up" done often.
Additional porting follows the same track. EVERY shop can improve on the existing port job, but will you ever see any gain?
"Just no radius on the intake" is what I was told recently be a shop on a set of heads I was having done, but since the car/engine was NOT a max, all out HP performer, it made no difference. We could always add a pound or 2 of boost if needed. 
Will you see any gain, probably not unless it is proven on the track. Flow bench numbers, like dyno numbers, do NOT tell you absolutely which is best, or even better, unless mods and changes are proven in the car under actual operating conditions. At least that is what the pros have stated.
You stated "brand new GN-1"s, so if the valve job is "no good" then you need to have your supplier make them right.
