Shower valve with separate diverter valve question

I'm building a shower with 1 main shower head and ceiling mounted rain head. I want the ability to turn either head on separately or both simultaneously. I'm using Kohler valves and trim.

Here is the set-up I'm thinking of:

Water pressure - stellar (not exactly sure what it is but can have two sinks running, flush the toilet and showering all at the same time and not experience a drop in pressure)
Water source: 1/2" copper
Kohler 304 pressure-balanced Rite Temp valve 1/2" inlets
Kohler 672 3 way diverter valve with 3/4" inlet and 1/2" outlets
Pipe between valves 1/2" copper using 3/4" to 1/2" reducers at diverter
Pipe between diverter and shower heads 1/2" copper
2 shower heads - 2.5gpm as required


Issue: Kohler said that the 672 diverter valve should not be used with the 304 since it has a 3/4" inlet, rather they recommend that I go with another valve (306 - three times the price of the 304) because it has a 3/4" inlet, and if I did go with the 304 and 672 I would notice a huge pressure drop.

Not knowing hydrodynamics, I thought I'd ask the pros if Kohler is indeed correct. Seems odd that the pressure would drop given the fact that the pipe used throughout would be 1/2".

What do you think, can I do what I intend without the pressure issue that Kohler claims will happen?

Thanks in advance!

why not keep the transfer valve at 1/2"

http://www.us.kohler.com/onlinecatalog/detail.jsp?item=274602&section=&category=&subcategory=


its a 3 way but can be used as 2

The trim to be installed does not fit on that valve. The trim is the Stillness series.

valve

The determining factor is not always the pipe size. Many valves have a very small outlet port, regardless of the pipe size it is connected to, so the water output is limited at that point. Check your Kohler valve to see what size hole is inside that 1/2" outlet port.

The Kohler technical specs don't indicate the size of the output hole on the diverter. Based on your question, it sounds like that would factor into the water pressure question - correct?

Same issues...

Look at two threads before yours. I guess instead of your ceiling mounted shower head, I'm installing a hand held next to my main one. I didn't realize there would be a problem with this. I just wanted to konw if I put the adapter at the main shower valve or do I put a reducer at the transfer valve?