The 450's will have the edge all day long mod for mod when it comes to power but the 400's are no joke and can be found pretty cheap these days. Unless you're looking to get serious into MX or drag racing I wouldn't worry about it. I ride with a few a 450's and my 400ex has never had a problem keeping up and it's just so damn rock solid. The down side to the 450's is they take a bit more maintenance then the 400's. Around hear you can pick up a nice 400 for less then 2k. In all fairness I do have Works front shocks and I did a little motor work but nothing wild. I wouldn't dream of trading my ex in for a 450 but I do know they are the "better" machines.
'02 Honda 416ex - A seasoned blend of 11 herbs and spices
'04 Honda Recon - The yard machine and snow plow
'88 Honda CR250R - Another chapter in my torrid all terrain affair