U2 clone: P4 to Q5 - questions

If I understand correctly, the Q5 is the most efficient Cree emitter currently available.
I'm thinking of using one of these to mod my DX U2 clone (this one), which shipped with a P4 in it.
Take apart head, unsolder old emitter, solder new emitter, add heatsink goop, done.
What worries me is this part from someone's review of the Q5:

"For those who plan to upgrade from a CREE P4 or Q2, and only plan to replace the emitter, don't be surprised that this Q5 may actually be dimmer. Keep in mind that the performance specs for these emitters are based on the current and must be driven properly. So the existing driver in your flashlight may not satisfy the higher voltage requirements of this Q5."

Is this true? Do I risk ending up with a light that's dimmer with the Q5 than with the original P4?

While I'm at it, a side question: how does the Q5 compare to the Rebel? Which is brighter at the same power consumption (say, three watts)? Are there differences in voltage requirements?

That's a tough question. It will depend on what the Vf is of that particular LED. Within each bin (Q5,Q4,Q2,...) there is a range of Vf(ex:3.7-4.1). Basically it comes down to the luck of the draw.

The last batch of Q5 emitters from that vendor had lower forward voltage values than previous orders I have received from that vendor and others. The last ones were all 3.54-3.56 Volt @ 1A, which is pretty good, I got the last batch in September/early October. Why not pop in a new driver when you are at it? Plenty of nice multimode or single mode drivers to be had out there.
Stefan

How does one check the Vf of their emitter? Just turn it on and put the multimeter on the +/- of the emitter and see what the voltage is?

My Q5 vF is [email protected] at the end my Q2 is brighter.