What exhaust is it and how much are they charging for fitting? If you want a bit more satisfying noise, why not buy just the backbox? Fitting charges should be less and youll get the same note as a cat back.
If its power you're after, put a decat and a flexi on, this will also make your standard cat back sound meaner while freeing up the two most restrictive parts of the exhaust! And if u dont have a K&N yet, throw one of them into the mix.
Backbox £150ish
Decat £75+
Flexi Pipe £90ish
All of the above are rough and for new prices.
I personally would choose decat & flexi as that will make your ZS sound proper beefy but still bareable with the standard exhaust and give you some very noticeable performance gains! I put a 300 quid cat back on, garage charged 50 for fitting and I noticed nothing performance wise! Sounded mint tho

I bought a piper decat for 120 and a mate fitted it for a tenner, and not only was the sound enhanced but the performance gains were awesome, it really freed up the rev range! My ST pulls all the way to 7k now in second and even pulls hard to the redline in 3rd! Best performance mod Ive done to date, not couting lowering as I consider that handling.