...my gauges work properly? Will my tach and speedo be accurate even if I don't change the insturment cluster?

 

yes.

 

Even though the tach on S4 only goes to 8000 and the tach on a S5 goes to 9000?

 

S5 turbo doesn't go to 9000rpm

 

I could have swore that I saw a S5 with a 9000 tach. Guess I was wrong.

 

A S5 N/A tach goes to 9000rmp cuz the redline was raised to 8000rpm, but the S5 turbos still have a 7000rpm redline, so they kept the 8000rpm Tach

 

The N/A's have a 9000 tach, tII's have an 8k. Not that it matters anyway, the trailing coil provides the tach signal, so it'll work all the way up until whatever the gauge says, and if you're revving past that in a turbo you should be shot

 

So S5 redline in a turbo is still 7000?

 

Yep