A) DBPro is much faster at both 3D and 2D. For 3D, you can expect between 5x and 10x the FPS of DBC (DarkBASIC Classic). For 2D sprites, you can expect 100x the speed of DBC. The 2D commands are generally much better, but be warned that the GET IMAGE command is much slower than it used to be.
B) Yes, you could make a MMORPG using DBP, but bear in mind that if you used DirectPlay multiplayer commands & Net Message commands, then you would be limited to 256 players. However, it is very unlikely that that many players will be either online at once or in the same area - so clever optimisations are possible. Multiplayer is pretty tricky in any language though. If you want more players then you could look for the TCP / UDT library for DBP - available from RGT: http://www.realgametools.net/forums.
C) We look forward to you and your creations
NOBODY has a forum name as stupid as Darth Shader. I do.