Logic itself shouldn't be taxing any modern Mac, it's probably the AU plugins (Logic's own and ones you've added) that are the real processor hogs. There seems to be a lot of uncertainty about how much of this workload can be moved to GPUs, and if you run a lot of obscure plug-ins it's likely that many of their the developers won't move them across anytime soon.
My advice would be to spend your money on more cores and stick with the D300 if you have to buy now, but if you can wait for the next update of Logic before buying, then do. Apple might have shifted it's heavier AUs to the GPU, heralding a golden age of GPU audio... or they might have moved none of them across, signalling that it's something that 3rd party developers are unlikely to bother with.
To tide yourself over, take a look at which plugins you have that are processor hogs, and see if you can use something that sounds as good but soaks up less CPU.