You are asking an incredibly broad question when you say "benefits".
There are a number of things that can be beneficial in breastfeeding, for the baby and the mother. So how much of a benefit will be obtained varies depending on what you are looking for here. Also, keep in mind that any "reduction" of risks, etc based on breastfeeding is only that. You can breastfeed 10 kids and one is very healthy, the next not so much, another has some issue breastmilk is said to reduce, you have issues maybe that were supposed to be offset by breastfeeding. At the end of the day, my point is, we all have to be realistic that there are statistical findings that are clinically significant but that doesn't mean we can truly prevent all things by breastfeeding. Likewise, if we cannot breastfeed exclusively, that doesn't inherently mean it has no value. If nothing else, it is safe, nutritious, incredibly portable when you are on the go & nearly free, so there is always that.
There are always antibodies in breastmilk, whether you give the baby one ounce a day or 25 ounces. Lactation in & of itself has been found to have potential medical benefits to women (such as lowering uterine cancer, possible breast cancer, etc).
I tend to look at such questions in life this way. Let's say ideally you want to walk 5 miles a day for health, but you are really only able to walk 2-3 times a week. Would it still be worthwhile those 2-3x times or no? What if it were only once a week?
The great news in life is that anything that is good for us is nearly never all or nothing. Usually you can find a way to balance it with what feels doable.
And as far as what exact benefits there are when feeding half & half, I do not think anyone being intellectually honest could tell you specifically what you do get in that ratio, but I can tell you it must make some difference, as I can link you to a study abstract that shows that breastfed infants (and it includes partially breastfed) show a larger thymic index & the thymus is critical in immune system development. https://www.ncbi.nlm.nih.gov/pubmed/8888912