Hi there,
I allow myself to answer as I do quite a lot of such headwork (although mostly on singles tt/xt etc).
See, if you put in vastly bigger valves in particular on the intake side in a somewhat space restricted confined combustion chamber, you can run into the phenomena that although you use bigger valves that you end up with less flow as the combustion chamber walls are to near to allow thorough breathing.
Considering now that the xs chambers can still imho be regarded as pretty old school (a hint like triumph) you are limited in which size valve you can stuff in in relation to diminishing returns and danger of valve collision.
E.g. imagine putting a 45 mm (could be actually with some reasonable surgery done) valve into the original head and see how much space you still have to the chamber walls and how much effort the engine will need to suck mixture through in comparison to before.
One usually can go in brief 3 ways:
1. Go with a reasonable size increment and watch that the increment stays within the duration of the cam and relief chamber by means of seat relief cutter for valve seat tool/stones etc.
2. Go with a bigger Increment and watch that valves do not collide during overlap and relief the chambers by milling machine or more heavy duty means
3. Go a lot bigger and reangle valves according to overlap necessities and port intake according to velocity profile needed.
Reshape combustion chamber according to what the flow characteristics and numbers dictate you.
Mostly that would imho only needed on 1-2 cams of which I could think of as otherwise one would get too much flow area at too much of semi low lift duration.
Hope that my short rantings made it a hint clearer.
Kind regards
Christian
PS: i do not understand what or why one needs a batch of xv valves done?
The size is pretty much the same and at most one needs to shim the springs according to needed retainer key height?
For chamber reliefing i'd reckon any machinist with porsche or Hemi experience or some valid capacity of reasoning/experience should be able to do such a seat job without any troubles?