2-5ms is not too bad imho. Then again, it also depends on how your application handles database calls.
More interesting though, might be looking at fault-tolerance, you might want to set up an identical solution in DC2, so you have a DB server in both, that way you can quickly recover from any disaster in one datacenter. A 2-5ms delay would be very appropriate for database replication, and if you use mySQL, you might even be able to set up multimaster replication and get better performance between web servers and nearest DB server
Though there are nothing wrong with pushing your service provider to commit to an acceptable level of performance, you also need to "face the facts". If the Datacenters are 300km apart, the first 2ms is simply due to the fact that not even fiberoptic WAN lines exceed speed of light (roughly 300.000km/s). The ping time is a roundtrip time, so 300km would in fact mean the time taken to travel 600km (300km each way). That takes 2ms at the speed of light, and no hosting provider can tamper with a physical constant like the speed of light