I'm sorry that I can't exactly help your situation, But I can say that I am having a very similar issue. I am running D800 AT&T, CloudyStock V2.0 with dr87's Simple Kernel V5. Only have one APN, which the the Phone APN. Now, I do receive LTE, it also shows up on my phone as 4G.
I have also changed that lil bit in the Build.prop to my destination Country, which is US. But, anyway, After all of this I can only get maybe 2mbps downloads and if i'm lucky I will pull 1mbps upload.
I know for a fact that the LTE in my town is not that slow. I'm not really sure what is going on either.
Before I had CloudyStock on my G2 I could get around 11-12mps downloads with 4-5mbps uploads.
Sorry that I can't really help you out OP. I need help as well.