I was contemplating putting a higher power motor for the y-axis and 2 same power motors for the other two axis.
Would there be any adverse effects?
There shouldnt been any problems EXCEPT that the motors might need different power supplies ie one motor is 3.2 v other is 5v well if you go 10 times the 3.2 you get 32 volts but that would only be 6.4 times the 5 volt and when runing steppers you need plenty of voltage for RPM but again as long as you treat the over size motor as one and the other motors as another then you should be able to get optimal performance from all. I have a question thought why over size the motor do some searching for formulas look for mariss in the forum and see his posts really good formulas that way you can skip the improperlly built machine thta almost everyone builds the first time ie 1/2 10 screws with 10 micostep gecko 203 and 269 oz motors that is what I have and after dealing with mariss I found out that this is the worst possible setup I could have chosen for what I want for starters it gives me a max travel speed that is fairlly low in the 120ipm range and it gives me like 800ftlbs of torque at the cutter well you really only need something like 100 for cutting almost everything except stone and metals so had Iknow that at the time I would have used 1/2 10 5 start which would have given me something like 300 to 400 ipm rapids so search the forum and study save your self the hassle of building being happy then needing tobuild again very soon.
Just my 2 cents.