If both AMD/Intel and Qualcomm do a good job with their core design and the same process node is used, I don’t see how a translation layer can be any faster than a CPU natively supporting the architecture. Any efficiency advantages ARM supposedly has over x86 architecturally will vanish in such a scenario.
I actually think the efficiency of these new Snapdragon chips is a bit overhyped, especially under sustained load scenarios (like gaming). Efficiency cores won’t do much for gaming, and their iGPU doesn’t seem like anything special.
We need a lot more testing with proper test setups. Currently, reviewers mostly test these chips and compare them against other chips in completely different devices with a different thermal solution and at different levels of power draw (TDP won’t help you much as it basically never matches actual power draw). Keep in mind the Snapdragon X Elite can be configured for up to “80W TDP”.
Burst performance from a Cinebench run doesn’t tell the real story and comparing runtimes for watching YouTube videos on supposedly similar laptops doesn’t even come close to representing battery life in a gaming scenario.
Give it a few years/generations and then maybe, but currently I’m pretty sure the 7840U comfortably stomps the X Elite in gaming scenarios with both being configured to a similar level of actual power draw. And the 7840U/8840U is AMD’s outgoing generation, their new (horribly named) chips should improve performance/watt by quite a bit.