The DET has the 30mm Exhaust valve for more low to mid lift exhaust flow. The DET 30mm Valve also uses a 7mm stem that is flared towards the valve head. This is for strength, heat dissipation, and also, these valves are sodium filled. The DE head with the 30mm valve flows better than the DET everywhere. The 29mm VE valve actually flows more than the DET/DE above .300" lift I believe. I would have to recheck but its pointless i think. The smaller valve allows higher gas velocity as it exits the cylinder as well as for a slight deshrouding on the back side of the valve which increases flow.
Remember, both the SR16VE and the SR20VE were designed to have LESS hp than any DET out there. Also a reason why the DET's have larger EX valves. The DE's have the same size EX valves as the DET because they are 95% the same motor as a DET. Rather than change the valves, valve seats, and so on, they just changed the guides and valves to 6mm stems.
-Ted