Yes, the tippmann 98 and all of its variants use the same threaded barrels.
The platinum comes with ATC, which helps to stop ball chopping.
Yes, the tippmann 98 and the tippmann custom are the same marker, therefore the threading of the barrel will be the same.
This statement is half true but half untrue. The tippmann A5 and 98 custom do indeed have different threads. But after the A5 tippmann made the X7 which has identical threads to that of an A5. Because of this a barrel with "Tippmann A5" threads, will fit on a tippmann x7, but will not fit on a tippmann 98 custom.
They should be universal.
No, only a Tippmann A-5 comes with a cyclone feed standard. However a Cyclone can be purchased that will fit the 98 Custom.
the barrel, hopper, stock, and many others.
They are actually one in the same. There was never an orginial "model 98" only the custom series. The difference though would be found in the Tippmann 98 custom Platinum series with its easy to remove body. There is also a difference in barrel sizes if im not mistaken but I think the newer 98 customs and platinums have the improved size (the same size as the A5)
probably up to 600 fps
standard .68 caliber.
They are both around the same performance. The J&J may be slightly better, being ceramic.