The pressure dial is typically the pressure in the radiators (central heating system) and isn't related to the domestic hot water supply.
There appear to be companies selling low flow shower heads that claim to reduce the amount of water required. Some examples here.
Your reasoning about the boiler reducing pressure on the hot water makes sense and I've noticed in my own house that the cold water is higher pressure than the hot, however I have no idea whether your method would make a noticeable difference. It'll be dependent on many factors: your cold water pressure, boiler specs, pipework diameter, type of shower, and preferred shower temperature at the very least.
