Plumtree's claim came in the wake of a bonus point victory over the Blues in Auckland in last weekend's Super 14 competition as the Durban-based outfit made it 16 games unbeaten in the Currie Cup and Super 14.
The British & Irish Lions face the Sharks at the ABSA Stadium on 10 June and Plumtree's men are shaping up nicely for that midweek encounter with an unbeaten run of four games in the Super 14.
"We are still not playing as good as we must. We have got the credentials to do well in the Super14, but that doesn't give you the right to win it," said Plumtree.
Having beaten the Blues 35-31, the Sharks move on to face the Queensland Reds in Brisbane this weekend.