If the employee violates the IP then sue him for that. No need for non-compete agreements. If you're worried the employee will steal your clients then you're not giving your clients a good deal at first place, and the employee shouldn't be prevented from working with your clients. This is a free market. Non compete agreements are completely anti-freedom of both enterprise and work.
> Company ABC brings you in, trains you on their know-how and core IP, which took a lot of struggle, $, and R&D to create - then you leave, go to a competitor, and effectively communicate and give away ABC Corps. most valuable asset.
Then the burden of proof should lie with the previous company, there shouldn't be any possibility of preemptive clause within the former contract. If it's clear corporate secrets were leaked then sue. If you don't want your employee go work somewhere else then compensate him, that's the problem here , most of the time there is no compensation.
It doesn't matter how much R&D was created. That's the role of patents to deal with that thing.