I don't have impact gun. Just arm strengthI did it with a jack and stands. It wasn't very fun, but it's possible.
I'd consider these mandatory tools/steps.
-Cowl goes off first (Before you jack the car) there's a DIY on that.
-Have an O2 socket handy (I got the taller thin-walled cylinder-type one from O'reily and it worked fine)
-Have good mechanic's set with extensions and universal joints. (An offset or double off-set ratchet could be helpful)
-Small flat head screw driver (For removing plastic fasteners in the Cowl.
-Impact gun (I use an 18v Milwaukee 450-lbs gun and it works great, even sheared a stud off of a friend's Fit) this will definitely help with removing the spring-tensioned bolts connecting lower OEM DP.
-A extendable magnet tool for removing/replacing/recovering those hard to reach bolts
-Ratcheting open ended wrenches 14mm/12mm/10mm IIRC.
And PATIENCE! Taking your time on this can easily take a couple to a few hours.
If you install a downpipe without a cat then defiantly no. If you install a downpipe with one highflow cat ceramic probably not but it will be closer than a metalic cat downpipe which will probably not pass an emission test either. Remember the stock downpipe has two large cats and the high flow catted downpipe has one that is made to flow first and foremost.