Nice bike!!
I still have one mediun session frame just laying around, freshly powdercoated too. I think it could be possible to convert it to 27,5 with little parts machined in between the rear triangle and the dropouts.
Or maybe I'm going to build it as 26" one day haha. That bike is so moto, I loved riding it.

 It will be a fast bike still with -2deg angleset and light parts, bearing in mind that it's a nuclear war proof, old school dh machine. I'd be happy to get it around 18kg.

 Despite being boat anchors, I love those Session frames. I also loved the short-lived Gary Fisher Diesel.

 Mafkees

 sooo sic !