So a few things:
I've seen higher level players opt to not upgrade past 30mil because of the heavy investment 300mil takes(1). That is exactly what is going on; up front cost for a payoff down the line. The thing is they have to upgrade every tile to 300 mil. An attacker only has to punch one hole.
Is that balanced? I don't know but it's important to keep in mind.
Nurfing walls/ramparts will do nothing for the imbalance between new players and old since there is already very little difference between two players in regards to walls (sans time upgrading) when taken on their own. Active defences and tower placement are king.
It will make a room take less time/investment to crack for sure but i'm not sure how much easier it will be to crack any given room unless you render walls/ramparts worthlessly weak.
Nerfing can also make it harder on newer players. Anything less than what will go down in a few creep lifetimes puts a massive amount of pressure on active defences to pick up the slack.
Hell I would argue that even if they where logged on at the start of an attack they couldn't write the code fast enough against a coordinated(2) attack. This is of course assuming they have the time to get the walls upgraded, newbies and all.
Put it at anything more than a few creep lifetimes and the players with well written defences won't even notice the difference against all but the most determined of attacks. And that's if they even bothered to build their defences all the way up anyway.
Changing upkeep amounts is the only workable suggestion i've seen so far. But strictly speaking a lot of defensive layouts have a lot of 'fat' that could be trimmed(3) and I feel there are unexplored strategies that could prove even more optimal than present(4).
We could just end up back where we where in the attack/defence with a new, dare I say, scapegoat. The problem is systemic and while walls are a part of the problem they are not the problem.
Use the extra GCL you have over them (due their massive standing investment) to level a couple more nukes at their room or something.
(1)Starting to lean towards this camp myself. But interestingly my code starts to break down for varying reasons if I where to try to use it.
(2)I'm talking your typical boosted squads. No matter how you slice it defensive code takes iterating and quite likely, boosts both take time. Many players simply do not work on defence until the enemy is at their gate.
(3)Solid lines of ramparts work well for some things but are not the most efficient design for energy use IMHO.
(4)Eg. New bunker designs, moving towers for walls out, exploiting unreleased mechanics etc. Actually I'm convinced that some combination of walls out & walls in simultaneously is the most optimal defensive (if not economical) solution.