well, one should use the more robust machine for the heavy lifting, which VE Pro and/or MIR Pro is going to be by far compared to the sequencer's load unless you're doing heavy audio work in it. if you have VE Pro 5 there isn't a lot of call for doing any FX in Logic. So in that scenario certainly go for Logic on the g5 machine.
If I really felt I needed MIR Pro and could raise the funds for it, I would do this: I would take the returning instrument channels from my first slave and port them to a third machine/second slave to VEP 5 as audio to instance of MIR Pro/VEP 5. IE: VEP5 as an insert or send from the sequencer to an instance of VE Pro 5 that does not host instruments, just audio channels with MIR Pro as an insert.
I will not recommend with any confidence, to foster a belief one is going to find joy on a single machine doing all three, say Logic, symphonic type of arrangments with virtual instruments and MIR Pro. MIR Pro uses SERIOUS resources. This is a little bit reminiscent of the gigastudio days, you just had a dedicated computer or ten for it.
