glens said most of what i would have said
as for TD vs Y vs XYZ its all very close, when playing around with NA on this car there isn't alot to be had.
i can recommend the invidia but as yet i don't know what its total outcome will be as its not going on the dyno till friday. i would say the new g300 version is better than my g200 as it uses an x pipe to scavenge between the pipes.
go with what you like
db are the b all and end all, its tone and resonance, you don't want something that booms in the cabin, you can have loud without boomy.
i had a custom built hayward and scott system on my old mr2 and with a TRD manifold it was about 105 db and was awesome, but crucially i didn't get any cabin boom. it was just loud
get to a meet and try them out, see what you like. just remember the exhausts sounds different once under load.