It sounds to me like someone is trying to pitch a landline to you which will really cost more than $5 additional a month, in an effort to either make a sale or as a part of some policy to kill dry loop DSL.
In short, if you're paying for Enhanced, you pay for anything up to 15Mbps as you've mentioned. Please keep working with Verizon, refusing the landline, since the problem is not a billing issue, but is a technical issue. If you run into problems or would rather speak with someone who I know can help, just let me know and we'll get this straightened out.
You should switch to WOW! internet. Faster, cheaper, and nicer people. Verizon has terrible customer service.