I did some tests with using PETG as support for PLA and vice versa. My findings:
using PETG as support for PLA prints works perfectly well. Even with flushing volumes of 280, 'though the PLA print has some PETG remains in the printed object, the PLA object is very strong. In my case the flushing volumes that work good are 280 from PLA to PETG and 346 from PETG to PLA.
On the other hand i had no luck using PLA as support for PETG prints. I used flushing volumes of 346 from PETG to PLA and 480 from PLA to PETG. The printed PETG object will crumble very easy in my hands when applying pressure. So for PETG object with PLA supports the flushing volume needs to be cranked up quite high. PETG contaminated with just a little PLA will result in poor layer adhesion.
For me, with these large flushing volumes, printing PETG objects with PLA supports is a no-go. It simply creates too much waste for the benefits of having clean undersides, compared to simply using PETG as supports in this case.
I will only consider using PETG as a support material for PLA prints, as PETG is way cheaper than the support filaments like Support W or PolySupport, etc.