Hi and welcome. I don't know anything about the mods you're looking at, but I will offer the thought your subtank with .5 ohm coils is about maxed out on the Kbox. If you want more cloud, you're going to need more tank and lower coils. Unfortunately, I don't know much about factory coil tanks either, I rebuild. If you want to think about building-- you can buy premade coil wires, if you like, and just install and wick them-- I can heartily recommend the Indulgence MT. That thing is a beast coiled down around .3ish and upwards if 50 watts. They make a companion, the Mutank, which has the option of build or factory coils, but the folks in another thread don't like the factory coils much.
Regarding the mods, I am unreasonably prejudiced against P4U equipment. I recall a time they had some QC problems on certain models and I have never gotten over it. There are a bunch of people around who like the gear though, and they are attractively priced. Those guys will know more than I about them. I can recommend the Sigeleis as a good brand name. The 100 or 150 watt models, with or without TC, would work for what you want. In another thread, there tons of folks who love the Wismec Reuleaux RX200, if you don't mind a 3 battery mod. One caveat on it: our friend
@coilburner dropped his and it broke. Finally, more expensive, but I have a 200 watt Snow Wolf and I love it.