Great motor. About 300hp, but needs to rev for it. It'll be a huge difference from the 4.0 which is all torque down low. Hell, just the TJ to JK/JKU alone is a huge difference in comfort, size, and drivability.
It is true 12-early 13s had head issues, but they warrantied those (might have even extended the warranty for head issues on those years, I don't remember).
Just a heads up, 12-present JKs had a few gear choices between 3.21, 3.73, and 4.10s (rubicon and possibly willys only).
Power wise, it'll be quick even with 3.21s. My sisters '16 JKU is stock and pretty fast. A friend has a '14 with a 4.5" lift, ton of armor and 37s and stock 3.21s (I've been telling him to regear...). He's pretty even if not a tad slower than my 3.7 powered KK with 31s and factory 3.73s.