Magnex do a system, but the twin square exit is the only option for the astra and its bloody awfull! Each square exit is rolled inwards down to a pea size hole and looks pants.
Remus are only 2" and have no center silencer, and belive me its awfull. Also the exits are chromium plated for extra shine which is great!.......Until it cracks and flakes and looks shit.
Pipper probably do some nice stuff, I had a piper exhaust on a corsa and it was very very well built and had a loverly subtle sound....not sure if and what they do for the astra.
Having looked round myself in the past, you will probably find the mass majority of manufactureres only make Back boxes and/or are only standard bore size.
Noticed regal are doing groupe A lexmaul systems in total vauxhall, but probably quite expensive knowing regal.
Still think that jetex looks good imho
