I think doing single leg extensions are fine as long as you use proper form and exercise both legs to avoid inbalances.However heavy extensions are not a particularly safe exercise on the joints in any form particularly in the lockout position where maximum stress is on the knee(so I would avoid the lockout full extension position if you do go heavy).There are better freeweight exercises to go heavy on for strength saving extensions for warming up or detailing.Have you considered just buying the less bulky metal weights to allow more weight on the notch?