BACKGROUND: 
It's important to consider memory usage of applications on ARM computers like the Raspberry Pi.  When programming with Python, there are several GUI choices.  A couple of the most popular are QT and TK.  The Raspberry Pi 2 and 3 are limited by 1-Gbyte RAM, and 32-Gbyte max USB memory storage, per stick.  They also have a much slower RISC (ARM) processor compared to popular desktop or laptop computers.  Still, it's "enough" to run applications, even many GUI applications at a time if they use conservative programming techniques.  I'm figuring that if a user stuck to TK based applications (Python-Tkinter-GUI) with the Raspberry Pi, then there wouldn't be nearly the number of difficulties.
Q: Does anyone have any statistics on this ... by using Tkinter instead of PyQT for GUI program development with the intended user being on a Raspberry Pi version 2 or 3 ...
Performance ratios, programming with Tkinter vs PyQT:

Size of Program in storage 
Size of Program executed in RAM
Speed of Application
tkinter based GUI should be smaller on the disk and RAM but has less capabilities and may not suite your needs depending on what you need. tkinter is best for a small, simple gui. it will have no problems running a fair sized document.
Memory savings of Python Tkinter GUI vs PyQT