The physically shorter wiring is doing basically nothing. That's not where any of the latency bottlenecks are for RAM. If it was physically on-die, like HBM, that'd be maybe different. But we're still talking regular LPDDR5 using off the shelf dram modules. The shorter wiring would potentially improve signal quality, but ground shields do that, too. And Apple isn't exceeding any specs on this (ie, it's not overclocked), so above average signal integrity isn't translating into any performance gains anyway.