Last time I looked Zotac warranty is void if/when you remove the stock cooler. You might get away with it only if Zotac don't use warranty/seals stickers.
I can only speak from personal experience but the EVGA customer service is great. The warranty is transferable so if you later decide to sell the card it will hold a little more value.
Watercooling is fun, it looks good and if your card is power/volt limited it can help a fair bit
If pushing for max clocks I'd say the best Ti out there at the moment is the EVGA Kingpin. Ridiculously expensive though.