Can't say I've ever heard of a car hitting a bump and coming out with a damaged radiator, especially on a car that is only 3 years old. Nevertheless, sounds like a good excuse to upgrade to one of the Koyorad units, they are built much better than the OEM radiator.
You'll need to purchase a flashing device to do that. The V1.2 is the most cost efficient option but many users go for the V2.0 with the touch screen display. If you are on a tight budget you can always find a nice used one but be smart about who you buy from and make sure it is unlocked...
I couldn't put a number on it for you but it's a pretty significant job. Stick with an OEM head gasket. The ARP Hayabusa kit plus the extra set of washers is a good solution. You will of course want to check your head for flatness once it comes off, it may need machining.
This may be helpful:
"Misfires and You."
a.k.a. How to diagnose your P0300 (301, 302, 303, 304) codes.
Plugs, coils, injectors, valve lash, head gasket, ringland. Those, in that order, are the most probable causes of misfire conditions on L15 engines.
Plugs are pretty self explanatory; I've...
With an aftermarket intake installed it is certainly possible he is hearing the factory bypass valve, I have heard it before myself.
If it's venting all boost, it should feel like all of your power goes away at once. It would be very noticeable. That being said, I don't know if there are any...
The factory ECU uses the bypass valve as a form of boost control. As long as it's not literally venting all of your boost pressure at that point (while still under acceleration) it is likely behaving as it should.