So I was messing around with hidden properties in the 0.9.905 beta and I found something super op for the chemical engines, massFlowRateOverride. Editing this parameter into a small positive number (eg. 1, 0.01) instead of its default negative will give you an insane increase in Delta V, ISP and burn time without changing the engine's thrust or really any other stat (except for power usage).
Before
After
There is no limit to what number you can put!
Combine massFlowRateOverride with other edited parameters such as thrustOverride and you can get the theoretical performance of an antimatter fueled engine or even nuclear salt water!
I may or may not have gotten 800,000 seconds of ISP. How.