OK, the good news!
My muffler shop modified the ART pipes to fit OEM metal ring gaskets between the pipes and OEM collectors - the rasp has gone (of course) and they say it won't leak anymore (because of the OEM rings)!
I also removed those brackets I had shown on the pic and it seems like it helped to reduce the drone inside of the cabin. The volume of the exhaust is the same but a little bit smoother and it seems like I have less vibrations now! Though I'm not sure if it's connected with these brackets, maybe it's just because I fixed the leak between the ART pipes and collectors (though it had to affect only rasp, but who knows)... Anyway I don't see why I should connect those brackets back, as the ART pipes are lighter than stock HFCs and my Borla is also lighter than the stock exhaust, so I hope it won't crack anywhere
Anyway, now I'm almost happy with my exhaust, maybe will add some dynamat materials to the trunk and that's all!