it is just a waste of space for a cab and the light your using a 400 does not have enough light intensity to penetrate very far so any bud sites over 2 ft away from the light will be small and not develop very well due to lack of light so your yield will suffer, and seeing how its in a cab really wouldnt recomend a larger light as you may not be able to control the temps, if it wasnt in a cab a larger light or even more than one light like 2, 600 or 2, 1000 for 2 large plants would work well.
by doing a vertical scrog with the 400 in a cooltube mounted vertically you are eliminating alot of your lights weakness by keeping it close to more budsites and increasing your yield. so you are using the height of the tall room to your advantage but even then you would still have plenty of room to grow 3-6 plants and take full advantage of mulitiple screens.
if you split the cab horizontally and made it smaller width and depth then you would not have all the extra height and could mount the light in a traditional way but you would probably be looking at using more plants to fill the space you would be looking at atleast 4-8 plants in a flat scrog or alot more in a sog 16-32.
the down side to scrog is obviously training which can take some time and would increase the cycle time in the flowering room to more than 10 weeks even more so on a large vertical scrog in a flat scrog you may not increase the time by much but increasing plant numbers would also help fill the screen faster.
but at any rate a 400 mounted in the top of an 8 ft cabinet is not a very good idea and you are wasting alot of space. would take that in to consideration when you build it, if you want to just grow a couple of large plants, wouldnt even bother using a cab at all and buy 1 or 2 larger lights.