In BCON Tuning-Motives, line 0x2 is what you need. Communal shower should be 275 (decimal), while the coloratura shower is 400.
ETA: just to explain more, that value seems to work this way.
Showers have only one BCON: Tuning - Motives; bathtubs have Tuning - Bathing, Tuning - Toddler Bath and Tuning - Bubble Bath. Combo plumbings have those of bathtub plus Tuning - Showering.
As a general rule, in bathtubs and combos, Toddlerbath tuning = bath tuning, bubblebath = -50 bath and in combos showering = +50 bath.
These are all the values I collected from standard plumbing:
SHOWERS:
communal 275
cheap 300
coloratura 400
BATHTUBS & COMBO
cheap combo
bath 200
toddler 200
bubble 150
shower 250
hubba tubba
bath 200
toddler 200
bubble 150
mid value tub
bath 300
toddler 300
bubble 250
wooden tub (oriental?)
bath 250
toddler 250
bubble 200
colonial
bath 300
toddler 300
bubble 250
colonial combo
bath 300
toddler 300
bubble 250
shower 400
HOTTUBS
wooden hottub 75
vaporware 75