My rotors are soon done and I am considering upping the game from 7" to
8" in front and from 6" to 7" in back. Are these spacers a good way to
go or is that playing with fire?

https://www.amazon.com/Avid-Post-Mou.../dp/B00F5I3P5E

What my bike has in front now looks like this but more round in the middle:

https://www.amazon.com/Avid-Post-Mou.../dp/B00F5I3P5E

No spacer in back (yet) and there I could probably just use the existing
front spacer. The brakes are Promax Decipher and I can't get an 8"
compatible post with cross-connector that has the right "belly shape" in
the middle. The Shimano and similar ones are too small.

--
Regards, Joerg

http://www.analogconsultants.com/