Actually I don't see how these committed values are useful. For full nodes, they will validate everything so these values are not useful. For SPV nodes, these values won't help them to judge if a block is oversize. They still need to download the whole block to verify the size, and even all previous txs to verify the nSigOp. The only way to generate compact proof is using a merkle-sum-tree (but merkle-sum-tree has the same problem mentioned: after a softfork the counting rules maybe changed)