288 GTO, F40, F50 or Enzo?

Only applies to the header pipes, not the exhaust itself. Exhaust back pressure is bad full stop. The concept of "tuned length" extractors and exhaust systems does not encourage back pressure.
Wait for Phil to add his 2c worth here.

 

Very true, back presuure bad.

Strictly speaking extractors 'extract' using a tuned length so that optimal RPM the pulses will create a low pressure zone just infront of the following pulse and encourage it out a little (as stephen said).

However, these pulses can also be used to pulse back enabling different cam timings (and longer openings/larger overlap) and more efficient filling and scavenging.