How does that compare dollar-for-dollar to running Tensorflow on a dedicated Linux cluster with GPUs?
I wouldn't be surprised if, with some clever programming, one could pipe data from disk directly to GPUs and back to disk without involving the CPUs that run user programs except for the thinnest slice of time.
But, considering what you paid for the mainframe, you probably have the budget to commission them to write drivers just for you (or to license their source and compile it yourself).
Btw given they do linux can they twist so that channel is partition into for that linux container. In the new world just driver through.