For best performance its best to go for fsb increase rather than just chucking up the multy. 200x12.5 will not perform anywhere near as well as 225x11 even tho its same mhz rating. Looks good that you have it at 2500Mhz but it just doesnt have the performance.
Yes you will need to raise the vcore to get the multy up. If your trying for 12x200 then start with 1.7v and see how it goes. If its stable see if it will go lower.
Have you tried to get the vcore lower at 200x11? Try to find the min vcore needed. It varys from chip to chip. My spare will do it at 1.525 but this one will do it at 1.5v
What msi board are you using? I used to have a delta2 but it didnt seem to like anything but a 11x multy no matter what vcore.
If i remember right the wire trick is to enable the higher multys over 14. Not what you want really. the is also the L12 wire mod which get the board to use the 200 romsip table instaed of 133 one and sometimes helps overclocking. Some modded bios have it built in as a soft mod.
edit: just read your spec. pc2700 memory will hold back your fsb ocing as you already know...time for upgrade? If you want a modded bios for the board goto www.socketaplus.com
and signup and ask for one there.