You can change the ratio between water and coolant some and it will help. However, I wouldn't get carried away with it because the coolant has the additives needed to lube the water pump and prevent corrosion.

I'd go 55/45 or 60/40 at the most.

I also live in the Valley and my '91 300E operates normally with temps no higher than 105. Generally it's under 100 if I'm moving. I'm running a 50/50 mix of MB coolant and distilled water.

IMHO, the thing that screws up cooling systems more than anything else is the use of tap water. I avoid it like the plague and never have problems with MB cooling systems. BMW's are another story