Can you give me your reasoning for this? What happens?

Why would dedicated connections be better (Because it would prevent....?)

Switching from MTS to dedicated would be quite easy (change to tnsnames on client) - but what impact would that have on the system / response times / connection times / SGA usage?
OK, imagine your web app users are washing their hands in the bathroom sink before they prepare your dinner, since you sold the utility room sink and kitchen sink on ebay. You route the washing machine drain into the bathroom sink and run a load. How will this affect your dinner time? Will measuring the amount of water flowing into the sewer help? Would routing the washing into it's own drain help your users so they don't have to wait to use the bathroom sink until the washing machine is done with it?

What I suspect is happening (and why you should look at those V$ tables I posted on cdos) is your web users response time is going way down while the bulkier operations hog the shared servers. This is probably being worsened by the redo issues. But if it is the problem, then all your testing of disk response and wait time stats and whatall is kind of pointless. Those things should be done after correcting the configuration to separate out OLTP and batch connections. You may then find the operations choke your redo log buffer or thrash your RBS tablespace or whatever. But if the phone isn't ringing off the hook because the users don't notice, maybe they won't even let you tune and call you ahiggly town hero. Or it really could be what everyone else says and I'm just talking out of my butt.

Note that you can use both MTS and dedicated servers, that is just controlled by the client connection. You can either put it in the connect string (if I'm not completely confused, the syntax is in the docs somewhere), or just have another TNSNAMES entry with a different name and a (SERVER=DEDICATED) line, and connect to that for the big jobs. Be real careful about backing up the tnsnames.ora before making such a cut-and-paste mod, some versions of Oracle are just psychotic about stray spaces.

As to switching everybody away from MTS, I couldn't say if that would benefit your configuration. However, my experience with similar configurations has often been, throw out MTS and everybody happy. YMMV