well a 0.3 ohm build is a sub-ohm build, its pretty low and it does entail really understanding the consequences and dangers involved. The batt upgrade is good but there are still things to look out for. And yes the top cap will get hot coz it does generate a lot of heat. Personally that does not fall under a standard build, I owuld have like it at around 0.4-0.5ohms, maybe one additional wrap each coil for peace of mind