It actually depends on your hardware. Some motherboards have problems with 8GB sticks. But assuming your hardware will handle it, Alvine is correct in that you will lose the advantage of dual channel and 1 stick will be around 5% slower than running 2 matched sticks.
Yeah you will lose the advantage of dual channel. That will mean a ~5% performance hit. It should run fine with an 8GB stick though. Some boards don't do well with 8GB sticks. A 2 x 4GB kit is your best option though.
Probably not but 2 x 4GB kits are generally cheaper than single 8GB sticks and 4GB sticks are generally timed lower ( faster ) as well and why would you lose 5% in performance you don't have to lose? So there is no real reason to buy a single 8GB stick. If you later wanted to add another 8GB it can be hard to get mismatched 8GB sticks to work with each other too.