Is a separate phono preamp worthwhile?

Separate phono stages are so much better than the cards used in amps that you would have a hard time finding one that will not be a huge improvement. Yes I know that's an expensive Mac and yes they probably say it has a really good phono stage. The thing is a phono stage has to deal with the weakest signal in all of audio. Even your relatively high output MM is still in millivolts. Line level is in volts. So not even close. The phono stage has to equalize 20dB just for RIAA, plus another 20 dB to reach line level. Any noise messes with this big time. That integrated has high voltage and makes all kinds of noise. Also the power supply is vitally important. Separate phono stage has it's own. Not even close. 

While any $500 phono stage will be a huge improvement there are some truly magnificent phono stages like the Decware ZP3 at about $1200 that will work just fine with your MM. If you move up to MC then either use a medium output cart and be okay or add a SUT to the Decware. Either way magnificent sound for very reasonable cost. Or go to the Herron VTPH2A which is more expensive but also more flexible with greater gain and able to handle MC or MM. I have the Herron and know it is superb. Decware is lifetime warranty, and Herron customer service is second to none. Others can help you find ones for less if that's what you want. The Herron is so good I would just get it and never look back.

Wait until you go to a moving coil.  The McIntosh MM inputs are considered quite good.

Not any separate phono stage would be better but great many, that's for sure. Unless it is something like Gryphon Diablo integrated's phono stage, separate is a way to go, and even then if you want higher level. Sure, one more pair of interconnects and power cord. Yet, in your case, I would wait because you don't know which MC you might choose. Cartridge first or both at the same time. I think, phono stage should be at least about twice as expensive as cartridge, especially with LOMC.