The S1 has an accumulator under the back near the spare wheel well.

Fuel pipe size isn't an issue to be honest. You'll get about 400bhp worth of fuel down there.

Get a proper MF2 mappable 5th injector that goes into the cross over pipe.

There is other options. You can up the fuel pressure, adjust the electromagnetic fuel pressure actuator if it's not already been done and have the metering unit resprung...