Lost shower pressure when switching from double handle to single
Does this person have the professional competence to know the difference between water pressure and flow? 

Because the new thing in showers is "low flow" showerheads (often by law).  Meanwhile, any 1-handle valve must have an anti-scald feature which throttles back the hot water.  It's hard to make that work over a wide range of flows. So of course they optimize for the low-flow heads common in new-construction.  With a "classic" head, it may come out limp-wristed.   

You can do a bucket and stopwatch test on yours and (with her cooperation) hers.  I bet the difference is significant.  

I prefer a nice deluge, but even more, I prefer a long shower. Low-flow showerheads are not some sad wimpy affair. They feel like normal showerheads.  If flow is too low, They can be a little jumpy when used with classic valves. 



The dirty secret behind the dial valve is they are being forced down the throat of the American public by government agencies to reduce the statistical occurrence of scalding in showers, that itself is on the rise due to government recommendations to set water heaters to scalding temperatures to kill bacteria which can cause Legionaire's disease. (Thanks Jim Stewart, for the knowledge, not the disease).  Sigh.  Pull one string, and three other things unravel.  In all fairness, Legionaire's is a new disease only first seen in the 80's and has taken quite some time to figure out.  

Or, do as many of us have done, and get an on-demand hot water heater set to a sane temperature... And keep your valves. 



Oh, and here's another trick.   The notion of drywalling up the shower valve is most decidedly non-Victorian.  In most Victorians I've lived in, on the wall behind the mixing valve, there's either a tasteful cabinet door with a latch, a closet, or a built-in bookshelf with a false back.  That makes it ridiculously easy to gain access to the mixing valve and tub drain linkage, which makes a swap a very practical DIY.