WP 7/8 + xna games by default set to 30 FPS, I would like to know If i set it to 60 FPS , any disadvntages / Performence issues / bugs / any other. becuase i always wants to play/develop the game in 60 fps.
Windows Phone 7 SDK sets XNA to 30 FPS because the screen on Windows Phone 7 Devices has a 30hz refresh rate. This means the screen refreshes at 30 times a second. If you are drawing 30 times a second and you refresh 30 times a second your at the optimal rate of smoothness for that device.
The reason most people aim for 60 (or on my gaming PC, 120) is because most monitors have a 60hz refresh rate (some are now 120hz). If your FPS is HIGHER than your refresh rate you won't notice see anything else except for possible an effect known as "Screen-Tearing" which is what happens when you render more frames in a second than your screen refreshes.
In other words imagine you draw to the screen two times and then your screen refreshes once, why did you bother drawing the second time? You waste battery life, cpu usage, and gpu usage when you render faster than the refresh rate of a device. So my advice to you if your sticking with XNA is that you stick with 30 FPS because the older devices won't get any benefit by having more frames rendered and if anything you'll get graphical anomalies like screen tearing.
If you plan to target higher-end (and newer) windows phone 8 devices, drop XNA, go the Direct3D route and use Microsoft's "DirectX Toolkit" because it includes XNA's "graphics functions" like spritebatch but in C++ instead of C#.
I hope this helps.