Cb Cb -
With passive networks, there no real way to get around overshoot/undershoot vs. rise/fall time. The averaging cap will always slow things down.
40 ns response time is pretty quick. The only idea I have to get fast response time and the averaging you need to to have some sort of active filter on the output. There's an example given in the AD8307 datasheet on page 20. In that example the log amp was going down to audio frequencies and still needed a somewhat fast response time. In the example given, it is able to achieve 100 ns 10% to 90% response time. That is still slower than what you need, but between operating at 70 MHz and using newer and faster op-amps, you may be able to get better than you 40 ns requirement.
