Never, even if its crosby, suter, malkin, tavares, stamkos, weber, ovy, etc. Using rudimentary math, it is just not efficient to allocate a large portion of the cap hit to one player.
Best offensive players always get shut down. Look at Crosby and Ovechkin against Montreal in 2009-10 season where he got shut down and boston against vancouver where the nucks' top two lines were rendered obsolete.
You can try to use the 2008-09 pens cup run as a counter argument except malkin had a 4mil cap hit, therefore able to allocate money in defense.
Stanley cup winning teams can have a strong #1 D, but depth is what earns that cup. I cannot think of an example where a defenseman was paid 20% of salary.
Allocating 20% of cap would only work in individual sports such as bball, or maybe soccer. Hockey is not really an individual sport. Players can bring so many dimensions to their game.
Also: Are posters not aware that one of the biggest problems the leafs continue to have is that they are extremely inefficient at allocating dollars? It is almost as if they do not correctly measure cap hits against performance.
Reference: Jeff Finger, Mike Komisarek, Tim Connolly, Colby Armstrong, Mikhail Grabovski (1mil too high, rather Grabo than clarkson). Leafs will get better results through prudent spending.