Is it worth swapping MTB front forks/wheel to ISO 584/27.5" from ISO559/26"? Is there going to be some roll over benefit? how about getting up to speed?   

I'd be using it at mount stromlo in Australia, everything except the black downhill...  

EDIT: I am specifically talking about the rim size, assuming that there would be the same tire width/depth. So therefore a larger rolling circumference on the front than the back   

Currently has a SRAM XO - 1 x 10 (11:32 cassette), front &amp; rear brakes etc
I assume I read this correctly that you want to replace the fork and wheel making the front a 27.5" and the rear will remain a 26"

If this is correct I don't think their would be any real benefit. There were several companies in the past that did similar things and don't any longer the 69er comes to mind which was a 26" in the back and a 29 in the front. 

Rollover will improve for the front tire yes, but more than likely with the different geometry you will likely be putting more weight on the front wheel which will nullify any roll over gains. Also climbing will become significantly more challenging.

It will also track through corners differently. I would recommend sticking with the same size wheels. I don't think you will see an significant gains in doing so, or any gains for that matter. If anything I would recommend a fork of the same size with more travel if that's what your worried about. Your legs and body are the most important suspension though.