Got an Ego8 and upgraded to a Cure 3+, Zick 2 and Shaft 4 barrel system.
The cure 3+ made a big difference to me in terms of paint chops (but there are those that haven't suffered many chops with a cure 2);
The Zick 2 was an improvement - kick was reduced, but of all my upgrades, this was made the least difference;
Tthe Shaft 4 kit made a big difference - the shot-on-shot consistency was much better and the accuracy (as much as a barrel makes a difference) was improved.
I've not shot a Sly, so can't really comment other than to say - try the Shaft 4 if you can.
In order of 'value':
1) Cure 3+
2) Shaft 4
3) Zick 2
Hope this helps