There are different kits. The X kit is a 60mm thick 360mm long rad. That's huge. The P kits have a 40mm thick rad, and the cheap kits have slimmer still. The better aios have 38ishmm thick rads, standard rads are closer to 25mm thick.
Figure a standard 360x25mm is good for @ 300w worth of heat, a 280x38 is good for @ 350w, the X360x60mm is upwards of 500w.
Actual temp performance will be highly dependent on the fans themselves, not so much the rad, but those Vardar fans are designed for that rad, you'd need something like Noctua IPPC 3000 to get better results (still debating on that).
Thing about water cooling isn't gaining the lowest temps possible. That's an air cooling newb thing. With water cooling it's all about maintaining a temp. The cpu/gpu could care less if it's 50°C or 70°C. Performance, longetivity, everything is below specs, so that's all that matters. So you just need rad size and ability to get you below safe specs, and keep you there no matter what load.
Ever watch water boil? Takes forever. That's how your loop works, the coolant temp is what's important, the lower it is the better it'll absorb the blocks outputs. So a cpu/gpu can jump around like crazy and it means nothing, their temps get reported about 2x per second. As long as all that jumping around doesn't go upwards of safety limits, you are good, the actual number is meaningless, just what it represents.
So is the X360 kit good? If you bought all those parts seperately, it'd cost quite a bit more, so value is good. Considering it'd cost about the same, for a lower performing loop with parts seperate, value is even better. It's an all inclusive kit. Chances of you forgetting to order 1 fitting? High.
Only downside is it's general fittings. In a custom job, if you wanted a 45° fitting instead, it's not there, just the fittings that come in the kit, you'd have to buy that 45 seperately.