Hi Phillip,
the FR 255 provides more than enough memory for RunPowerModel (I just checked). It was the FR 245 which in its non-music version had this limitation, such as many other older watches (but even the non-pro Fenix 6 if I remember correctly). With all the newer Forerunners and Fenix models this is not anymore a problem. And it even seems that the latest software update of the FR 255 now brings wrist-based running dynamics, which should improve the results of RunPowerModel in case no external sensor is connected.
I hope this helps.
Cheers,
Markus