Your table is old and the wood is cut at a different angle from the factory that is not compatable with modern super speed cushions.
A GC4 should work fine with the currently available SS cushions, without any adjustment necessary.
I don't have first hand knowledge of the GC rails manufacture, but I would imagine they are cut in long lengths on a huge wood shaper, that cuts the final profile in one setup. Then cut to length for each rail segment. That type of manufacturing process should lead to extremely consistent rails both within each table, and from table to table. That's the same way wood molding is made. If you get the cheap ass wood molding from home depot for 99 cents, it will still match extremely well from piece to piece. I just can't imagine Brunswick would not know how to make the wood rails on their tables match.
If they don't match for some reason, I think something else is going on. I'd have to see it to believe it, frankly.
Its one thing to do a 50 yr old table where the modern rubber does not fit. Its another thing to cut the wood on a modern GC4 or GC5. I personally think its correcting a "problem" that does not exist.
sounds good in theory but it overlooks the fact that the currently available Superspeed cushions which are now made in China get hard spots in approx 12 months. It has happened to me twice.
I posted about this previously and there were a number of others who chimed in saying they have experienced the same problem.
It reminds me of how when I go to home depot and get anywhere near the garden hose or replacement lawn tractor tire aisles, there is a toxic stench in the air from these products.
Makes me wonder where are they sourcing the rubber they use, Chernobyl?

If I have a choice of getting rubber rail cushions mfg'd either in China or Germany, I will choose Germany 100% of the time.
Unfortunately, the wonderful Brunswick cushions from long ago are no longer available.
best,
brian kc