Suggestions for slow Internet

Our company has bonded T1's that amount to about 4-5 Mbps Down/Up.  We have about 20-25 people on this connection at any given time. When it's a full house you can expect that speed to drop to 2-3 and it starts to crawl.

 We are a predominantly web based company in that we use a web app as our main business app, a hosted Exchange, and then  the usual other web traffic.  I'm talking with my boss because we've had some complaints recently about slow down and I'd like to help bring resolution to it.  IMO we need a faster internet service but I know there's also some qos stuff I could do to give priority to our web app and hosted exchange (although I suspect the people complaining about speed are trying to use FB ;)  

We have Sonicwall 2400 appliance and 2 static IP's that connect via 1 cable from the ISP's box.  My boss has set up rules to send some LAN IP's downstream to our second IP but I'm not really sure that's helping since it's still going through 1 cable. Need some advice?!