If we are talking about the peak of both civilization (pre-fall Eldar) where the webway was fully operational I don't see the imperials lasting. The Eldar commanded the stars, good bye sun, farewall Mars and Terra. Now if we assume that that quote is nothing but boasting, they still owned most of the known galaxy. Resources, technology, logistics, speed. The only real question is how much Damage the Imperium would do, before Eldar arrogance was humbled enough to pay them real attention.