Rear extenders do look like they've been trimmed.
http://www.hpashop.com/Ohlins-RT-E9x...-extenders.htm
Mine are original length and they don't protrude very far beyond the trunk liner. When the suspension is unloaded (e.g. up on jackstands) they are flush up against the liner so I'd suspect shortened extenders might pose a problem in this scenario.