> How much have legal costs to the client been reduced due to this increase in efficiency?

It's pretty hard to answer that one as it depends on the type of legal services. Anecdata only...

My experience is from a UK based perspective. Over the last decade I reckon the likelihood of using an external firm for standard commercial work (sales contracts/standard employment) has dropped dramatically (80-90%). There are lots of reasonably trained lawyers coming out of the large firms, so you can hire an internal resource or use a boutique firm. They can do almost all that work.

International work remains the bread and butter of the large firms. Where you're dealing in jurisdictions that you're uncomfortable or your internal resources are not trained for. Super large deals where (frankly) the insurance from the law firm is worth it. Or specialist matters (complex IP) where the business risk is too significant.

Litigation is a different matter. No fundamental difference there in my experience. Perhaps on the administration side, but that's often a rounding error compared to the costs of the partners/counsel.

Someone said earlier that 'commercial management' has improved. As a non-legal specialist managing lawyers that is definitely a cultural change that the legal profession is embracing but still adapting to.