What kind of additional overclocking are people expectting from custom cards versus founder edition?
Would like to get a 1080 Ti before Mass Effect
Voltage has almost 0 effect on Pascal. So the AIB is more about getting better cooling for lower noise output. The cooler you keep the card, the more stable the boost overclock becomes. For example, my default cooler on my 1080 I could never reach 2100 Mhz and was generally only in the high 1900s because the temps were 75-79 C under load, but when I switched to a hybrid cooler with 50 C max temps I can hit up to 2120 Mhz and it always stays above 2050, typically above 2100.
For any card launching tomorrow, it will either be using the default Nvidia cooler or the same exact cooler they used on their 1070/1080 line, while the Ti card has much higher power draw/usage. It would be best to get a card with cooling actually designed for the 1080 Ti (cheapest option), or go with a hybrid water cooler (more expensive option). Alternatively, you can get the stock Nvidia reference design/cooler and if you are dissatisfied with the performance for cooling/noise, you can replace the default cooler with an aftermarket AIO hybrid water cooler kit like I did.