I stuffed Mk XII gear on my NX along with quad cannons up front and she packed quite a punch for a NX but her low hull and limited abilities makes her a wet paperbag when it comes to strength.
I am or was till internet was cancled, working out trickling down gear from my personal flagship to older ships as I got upgraded gear for her. The better gear does help the lower tier ships but its all wasted effort in my opinion considering your abilities are limited and can't use tank style buff setups. But its still fun to do and put the ship up against stinger enemies if your seeking a challenge like I do.
I did the same a few months back to play around. To see what happens. I loaded my Connie Refit and headed out. Went to do the explore 3 systems in the nearest cluster. I did well on 1 set of fights. They was long but I managed to take out the ships. The next one was a Romulan one. I barely made it through until I got the Warbird at the end. I got him down to 1/2 health a few times before giving up. Just not enough hull and dps to down it.