Q. Is it stable in high speed or does it shake a lot - Thunderbird 500X?

There is alot of difference between shaking and vibration. bike vibrates alot but manageable. but those will be sever after 120(even with proper riding gears). however bike's stability on road is pretty strong. bike won't wobble at high-speed

Cut off @ 138..i simply reached 135 on open highways.. But speed comes @ a great cost.. Oil. Always check oil every 750 kms.. Service book will tell you that 2.7 ltrs the limit but this is wrong info and it is targeted to cause engine complaints within 2 years.. I would suggest motul synthetic oil instead of RE oil..

For me with proper riding gear even at 120km/hr i found it stable above that yes its vibrating. And without gear 90~100km is ok but you cannot sustain the speed.