Marantz TT-15S1 anyone with experience fitting an electronic speed box?

Look for a used Phoenix RoadRunner unit. Best to check before however, if you can fit the speed sensor underneath the platter and your AC motor will work with it. 

The company is now defunct, but I see listings now and then.

https://www.lpgear.com/product/RRDIGTTTAC.html

 The Clearaudio controller isn't on the same level as a Phoenix.
It appears it simply delivers steady power to the motor. There isn't a speed monitoring circuit telling the unit to control the speed of the platter. How is that supposed to be accurate? 

If your table run a little faster or slower, a speed controller can correct it, another benefit is you don't need to move the belt/pulley for 33.3 to 45, just push a button like a DD table.
a guy from stevehoffman forum use a Project speed box with Marantz TT-15 :

http://forums.stevehoffman.tv/threads/turntable-speed-controller-do-you-use-one.562398/page-3

But the Marantz motor is 5Watts and the Project box is rated up to 3Watts, not sure it actually work

Also, I have the Phoenix, which offers a much finer speed adjustment than the Speed Box.  it's not just an intellectual exercise - I found I could not get the speed perfect using the coarse .1 increment found on the others and needed to go to the finer adjustment to get it dead on.  
I like the Phoenix a lot, BTW.  But then again, my TT started running very fast for some reason, so I needed an outboard box of some sort. 

Chayro, I know all of this already, please read the original post thanks.

Ok - then I suggest you buy one and listen for yourself.