I have had good experiences with the CE-2 cartomisers, but I've only used them at 3.7V.
well you could always remove the inner refill plug and fill it like that then place the inner plug backwards, so its easier to remove and install. or you can use the center plug.
the r3 u tried , did u check the ohms, many people are using the lower ohm e2 at 2.5 or 2.6 and those get hot and cuase a dry taste at 3.7 volt.
now you can choose higher ohms when buying the e2.
I've tried the R3 in 510 and M40x types and found them to be terrible. Fiddly as hell to fill up and develop a burnt taste after the first 5 drags that doesn't go away. I wanted them to work but they just don't. They simply don't wick fast enough and are clearly a beta product.
I'll give the R4s a go when they reach the UK, but I've lost some of my enthusiasm for cartos to be honest. The R3s have left a bad taste in my mouth (literally).
Even if they improve the performance the filling process is a PITA. I don't really see why I can't just pull off the ring, drip inside, and replace it. Who wants to faff around with a syringe and a bendy plastic tube trying to stick it through a couple of microscopic holes. Yeurgh...