I've found the same for low temps in the EP-330 -- the low with one burner is 275-300. The problem is the great stuff they put in to get a more efficient grill (lined hood, bottom heat reflectors). Great for propane efficiency, not so much for low temps.
And, BTW, indirect is anything NOT over flame, so with only one burner going, you still get reflected, radiant, convection heating just fine. I do this all the time even with the sear station going (for something like baked potatoes).
Andy I like the idea about propping the hood, but... On my 330, I lined the front and sides with wood stove gasket tape to prevent the intense leakage of smoke (from wood chunks) all around the edges and to specifically direct the smoke out the back vent gap. This is great to improve the "smoking" ability of Genesis. Since I use wood for grilling more often than not, that was important to me.
My older model E-310 would go as low as 250* with just the front burner, but the EP-330 has shorter burners, so you might otherwise expect lower temps. Then the "efficiency" comes in.
So using one side burner is the right answer for lower and slower. As David said, rotate your meat just in case, but also remember that the temp shown is for the whole grill. It will be hotter over the operating burner, but a little lower than the thermo over the other side (try setting an oven thermometer inside). And I haven't had any real problems with meat cooking more in the flame direction when it is on the "inactive" side of the grill.
Edit: Incidentally, I was glad to see your temp for two end burners going on low. Mine is the same 325*.
Rich