I think you will be just fine with what you have hanging. 8 cobs will do much better than 4 with twice the wattage.
Yea i would like to pack the tent out with 7 autos really...even if they are small just run a SOG kinda style in there just to see if it will work with the new light, im hoping that because of the layout ive gone for the spread around the tent will be nice and even allowing for them to all get even light...which I think was an issue running one HID in there, it just didnt get to all the little bits around each plant properly, especially being a 250W....i might even get 4 x citizens 1212's and some cheap drivers off ebay which would probably cost another 75 pounds with heat sinks which would give an extra 15,580 Lumens and would add another 326 PPFD to the 750...but i think thats overkill in some respects! I have another Meanwell HLG-185-C700A driver at home that i ordered by mistake so I could just run another 8 and have the sun in my tent
Im just trying to get my head around those figures and that graph and Im going to try to convert it to an actual % figure...only as the PPFD figure from the calculator would be based upon your area of space entered, where as that PPFD on there graph could be based upon any size or area....on the COB calculator citizens are down at around 51% where as my CREE's are at 64%...i know its only a slight difference but I definitely think it will be noticeable in the long run...I might even make a cheap Citizen setup on the spare driver ive got, even if its for a VEG tent or for side lighting...