What is the real-world throughput difference between the 1500 and 1900 Spark models?

The datasheet numbers are pure fantasy. They always are.

Looking at the Spark 1500 vs 1900 models. The spec sheet shows a big jump in "Threat Prevention" throughput. In reality, what does that actually buy you once you turn on the necessary inspection blades?

* Is the 1900 just a 1500 with more RAM, or a different CPU architecture?
* Real-world drop-off with IPS, SSL inspection, and Anti-Bot/Anti-Virus enabled?
* Anyone have experience with support renewal costs on these? The 1900 is bound to have a higher software subscription price.

The throughput after all the mandatory features is the only number that matters. Not the firewall-only lab test.