I found this ad in ebay Z80 6 mhz

Would this chip fit in my nms8280 or vg8235 msx2 ?I thought having a faster processor would be cool.Is there a way to clock this back to 3.57 if this upgrade work?

Yes, this IC will work. But you will not see a difference at all, since the clock sent to the Z80 is the same.
This 6 MHz is very common and quite cheap.

To get a real speed improvement you will need the 8 MHz Z80 and change the clock.
See this page for socalled 7 Mhz clock circuits, the Superturbo circuit is made up of standard components and easy to assemble.