COS 3 or COS 5 for a 2000 corvette?

I converted a 1999 firebird to a COS5 for a 3bar set-up. Was a pain since I could not use the 1999 OS, but cut and paste was my friend. Worked very well.

Now working on a 2000 Vette for a friend. Can't use his 2000 OS so it will be cut and paste time again.

Are there any "gotcha's to look out for with ETC?

On the EFI Live COS vs OS it looks like I'll have to use COS3 and a 2002 ZO6 OS. Any downsides to COS3 vs COS5?

Thanks for the help.