What would happen to the amount over 17k?
Nothing would happen to it. Most likely, your employer would stop stuffing money in your 401(k) account once you hit the 17k limit, and deliver it to you in your paycheck. I think I have heard of (rare?) cases where the contributions don't automatically stop once the 17k limit is reached, but in that case the excess contributions still have to be yanked out of the 401(k) and returned to you at some point to correct the error, and then you have to pay the taxes (and perhaps even a penalty?) I'm going to max out my contributions by next month, so I plan to get in touch with the benefits people at my company to see exactly what happens in our plan.
You most likely could contribute after-tax money to your 401(k) if you wanted to stuff more money in there, but you have to explicitly select that option, and I think it's pretty rare to find cases where it's obviously beneficial to do that.
The match is a fixed percent per pay period, not a fixed percent of annual salary. In order to get full match, I have to contribute up to the full match amount every pay period.
I've mentioned before that some plans (including at least one I participated in) do
a true-up contribution to make up for the "penalty" that you pay in reduced matching funds when you hit the 401(k) max before the end of the year. I don't know how common this is; it's another thing to ask your plan administrator about, but if your plan does it then you can go for an early max-out without worrying about leaving money on the table (though the true-up only comes after the end of the contribution year).