Maybe the endplate vanes work best with a rear blown diffuser?
Maybe fixing the reliability issues did increase car performance due to it being run in compromised configurations?
The team had KERS and rear wing issues early on. Both were fixed, both were performance issues.
I don't think it is as easy as slap on this part, slap on that part and go.
On the bright side, the big update in Silverstone worked right out of the box. That says a lot for a team that struggled with updates last year. Many teams are struggling with updates this year.