1tb OneDrive, 1tb external SSD and my internal 512gb works for me. Why would you 'hack' such a nice piece of engineering?
I certainly wouldn't hack my sp3 as its too beautiful to break! Even though its behind the kickstand, I wouldn't risk it, but that person who did it certainly got some balls!. I bet the air flow around the laptop is probably effected and maybe the heat of those mSSD are hotter(?)
external 1tb ssd via usb? I'll have to look into that as I didn't think usb would be capable of achieving the speed of what ssd is capable of. I suppose if you don't want to lug the external drive and no access to the one drive cloud, then internal 1tb would be handy, but still, its not worth the risk.