The MacBook will run great in 'clamshell mode', it is designed to operate that way if needed. I use it like that all the time, to me it's the best of both the desktop and laptop worlds. And indeed, SSD is much faster than the Fusion drive in some cases, that does make a difference.
MacBook can be use in 'clamshell mode' only occasionally or also as extensive mode?
I did remember that on older model I wasn't recommended because it could cause over heating