Thing is, you can fit two 140mm fans instead of the 200mm one in most cases, which gives better air flow at lower noise levels (my experience anyway). But since you have the 200mm ones already it might be a tad expensive.
MX-2 shouldn't cause any problems. And tbh 30° above ambient doesn't seem implausible. When I still used my stock heatsink I was idling at +/-48°c with an ambient of about 20°c. That was a stock i7 860 though, so I don't really know how they compare. Now I am using a BeQuiet Dark Rock Pro 2 and during prime95 it barely hits 48°c on 35% fan speed. So yes, it is worth upgrading!
And yes, delidding will void your warranty...
I would add high ambient temps, bad case airflow and the general sucking of the stock heatsink to that list Also, what TIM are you using? (probably won't make a very big difference, unless you are using a really bad TIM)
The Thermalright TY-140 and the Corsair are probable the best 140mm fans. The only white-bladed 140mm fan I know of is the white version of the Bitfenix Spectre (Pro) fan series, but those are not quiet.
I have read both actually And I'm just saying this, because he probably wants 200mm because they are supposedly quieter, which isn't the case. And you won't have any problems when mounting them horizontally right away, they just dry up way faster. And yes, those Bitfenix fans should fit in the Corsair 600t (only the NZXT cases have differently sized holes afaik.
I personally prefer 140/120mm fans for a silent build. Mainly because there aren't any really good 200mm fans. Those Bitfenix fans also have sleeve bearings so they won't work properly when mounted horizontally and will need lubricating every once in a while. Only 200mm fan I know of with a non-sleeve bearing is the Antec Big Boy, which uses a ball-bearing, and in turn is quite loud.
Depends on whether or not something is blocking the intake side of the fan (like a fan filter). If something is directly against the intake side of the fan, you do not want the eLoops. If not then go for the eLoops. Just their general sexiness is enough to recommend them
If you want a silent pc, neither the Bitfenix nor the Sickleflow fans are a good choice. Apart of the Bitfenix 120mm fan all the fans in both series are quite loud, and the 120mm Bitfenix barely pushes air. On top of thay they both got sleeve bearings, so they will get louder over time. Only good silent fan I can think of with leds are the Enermax fans.