Actually the two bullets are exactly the same length. The difference is the radius of the ogive. The .30-30 version has a smaller radius ogive and a more blunt profile because the case is too long to allow a better ogive profile to function through the action. The correct FTX bullet for the application doesn't intrude on the powder space any more than a normal round-nose or flat point bullet. Hornady doesn't recommend the FTX (MX) for the .30-30 because it will not function through the action. The M94AE has the same problem with that bullet in a .307.The FTX for the .308 MX is longer if I recall. This leaves to little case to hold powder in the 30-30. I believe that Hornady even mentions not for 30-30. Yes, they are .308 in caliber. So you're right there, but it isn't recommended.
As far as opening too fast, the .307, as mentioned above, has a thicker case and is limited to a slightly lower pressure than the .308 because of the action that it was designed for. As such it isn't capable of .308 velocity and the .308MX is slightly faster with the same bullet. It operates well within the design velocity of the FTX bullet.