At some point i be upgrading muffler/back box. saw these
Ebay Blitz muffler for Altezza
&
Ebay HKS silent hi power

Good brands but think bit on expensive side equivalent of AUD 660 anyone think so ? or am i better off buying a universal HKS muffler ? i did that with my 03 Corolla VVTLi , japanese brand muffler for the JDM corolla runx didnt fit the UKDM / euro models, even with buying, fitting/welding of the universal HKS muffler that still worked out cheaper than car specific HKS/Blitz muffler like the links above.

bad points are that i couldnt fit my original muffler when i came to sell car & the universal HKS mufler was loud !! the advert suggests the Blitz muffler isnt too loud!!

What would you do or had done?

I would look for a second hand exhaust if cost was a problem. I'm also looking for an exhaust too. The HKS Hi-Power Silent seems to be where everyone is pointing at. I think your best bet is to look online possibly try the Japan auction websites?