the new VTEC3700 "Improved" are available with immediate effect, basically we ship them from now.
To answer your questions:
Now, are these the same size as the 3800 "FAT BOY" or the original 3700UP?
-> same size as original VTEC 3700 of course.
And performance wise, are they closer to the original 3700UP or to the new 3800?
-> about in between the "old" 3700 and the 3800!
And finally, when LRP announced the 3800 "FAT BOY", what did they not simply name it 3700 "FAT BOY" so that it could be legal anyway.
-> that wouldn't make it legal for sanctioned racing... the cell design/size is of course homologated and can't/shouldn't be changed!