More to the point, how much faster will you be on your MTB from putting in the road miles 🙂
On the MTB I carry way more kit than I do on the road bike – and it gets used – spare tubes, toolkit, food etc.
On the road bike everything I need fits into my shirt pockets + 2 bottles on the bike.
You don’t need anything like the amount of kit on a roadbike that you need on an MTB a) much less goes wrong b) you’re not in the middle of the woods if it does so you can just limp back to the nearest cab/train station…
I suspect a heavy pack on your back for the position you’re in and for the length of time you ride a road bike for, will do your back no good at all.
A couple of spare tubes, wallet, phone, pump and food will fit into your shirt pockets and you’ll be right
IME roadie comments are 99% tongue in cheek