E30 Fuel tank (expanded capacity)

[blueapplesoda]

technically you could run boost...

You'd get some wicked fast boost response haha, but you're limited (somewhat) to low boost

unless you're running race fuel all day.

[briansjacobs]

I only run 110 sunoco GT. AFR is between 12.3 and 12.8 from 2500RPM to redline!

[Todd Black 88]

There was an option on some euro M3's to have an aux tank in the trunk. This is what your OP was refering to I assume?
Anyways...yes i did exist, and I have seen them. IIRC all sport evos had them.

Link to part number

http://bmwfans.info/parts/catalog/E30/2-doors/Europe/M3-S14/LHD/M/1990/browse/fuel_supply/additional_fuel_tank/