Replacement hardware kits?

For some of the hardware used on RZR’s, why are those tapered self-threading style bolts used instead of normal straight bolts like the Japanese brands use? I noticed some of the OEM replacement chassis/frame parts that you order don’t have threads in the holes, you can just use the tapered bolts to make the threads. Is it better to use actual thread taps instead, then use regular straight bolts wherever possible? (newb questions – recently got my first RZR)

Are there any kind of aftermarket/higher-quality/better looking hardware kits available for RZRs (XP1000), or is a hardware store the best bet?
And which hardware/bolts on the XP1000 do you think should be upgraded for better quality/strength and looks?