Just incase anyone ever wanted closure to this one, I went with standard sized speakers in the standard location.
The rear quarter plastic is thick enough and has enough curves to be strong and have 'little' vibration. Plus the angle of putting the speaker in the metal body means it's firing at the wrong angle and firing into the rear quarter void. I felt the standard size and location would offer best results.
Spent a fair amount of time and money on vibration damping and sound deadening and a new set of speakers for the front. Rewired the entire system, amp and a teeny sub.
The sound is amazing, it's brought to light a few rattles that need sorting now. But it's the same thing as bhp, I'm already plotting a bigger sub and amp to match the quality and power of the main cabin speakers.