What kind of performance can I expect from Mid 2015 Macbook pro connected to two external 4k displays -- I mostly do development?
EveryMac.com reports that:

This model can support a single external display up to 5120x2160 at 60
Hz via Thunderbolt 2. Via HDMI video output, it can support 1080p up
to 60 Hz, 3840x2160 at 30 Hz, and 4096x2160 at 24 Hz.

So you could have one 4K via Thunderbolt and one 4K via HDMI.
Two 4K displays are at least within spec, but whether it can 'handle' them, in terms of heat generation/dissipation and performance load, while performing other tasks, remains to be seen. (Discrete GPUs on MacBook Pros have a chequered history in terms of reliability.)