4mm? Wow!
I don't know if the piston rods and trigger mechanisms are the same when comparing the LGV and Century if considering sleeving down? But replacing that huge spring with something more modest has to be a good place to start. Replacing the piston weight with a lighter top hat could well help, too, but is, obviously, also dependant on the weight of the piston itself. There's many a sweet shooting tune been achieved on the HW80/35 whilst retaining the original bore and, obviously, a less involved conversion. I think a satisfactory tune could be achieved whilst keeping to the 30mm and just addressing that spring and fitting some snug front and rear guides.