i've heard of some people with five thumbs on every hand and they screwed up the memory upgrade on a mac mini - but if you have a healthy hand-eye-coordination and hande your mini with the needed care, your waranty is not in danger
There is not even a tool needed. Just open your mini and change the RAM. It doesn't take a single minute.
the RAM modules apple is using are the same modules you can buy at amazon or any other dealer - rember, it's all the same intel platform and the memory controller is embedded into the i5/i7 processor - thats why every DDR3 RAM will perfectly work in your mac mini
there are no faster (lower latency) DDR3-1600 modules available as you can buy on free market, not even for apple
so do you a favor: save up to 230$ and upgrade the RAM yourself
use the money to buy your mini with a fusion drive - this can also be done by yourself (a singe 128GB SSD costs not more than 100$ - instead of 250$ if you buy a fusion drive) but the upgrade and hard disk preparation is quite more difficult than the very easy RAM upgrade on a mac mini