First question: why do you feel that you need to quantify this? Are you being asked to provide metrics? Would a qualitative evaluation be more valuable here?

It also sounds to me like you're focusing at the wrong level - i.e. on too small a group: it's not so much about making the testers more effective, or the developers more effective, but about making the team as a whole more effective. Can you get feedback from the developers about how it's helped them?

I can't write an answer about metrics without linking Cem Kaner's paper on metrics.

First question: why do you feel that you need to quantify this? Are you being asked to provide metrics? Would a qualitative evaluation be more valuable here?

It also sounds to me like you're focusing at the wrong level - i.e. on too small a group: it's not so much about making the testers more effective, or the developers more effective, but about making the team as a whole more effective. Can you get feedback from the developers about how it's helped them? Is there anything that it's allowed you to do that you wouldn't have been able to do before (turn around an urgent change in 2 days without having to spend 3 weeks on regression testing, for example, or add functionality partway through a project which let you keep/win a major customer), that would be appealing to your stakeholders?

I can't write an answer about metrics without linking Cem Kaner's paper on metrics.