Can XR-E and XR-C LED's be combined on the same string?
Yes, given the right conditions. Any LEDs that can take the same drive current can be combined in any given string. So, for instance, if your driver provides 350mA, you can put XR-E, XP-G, Rebels, whatever on it. If your driver provides 1500mA, you better not be putting XR-E on it!
Just a few specs would be nice. I believe steve is just a fellow reefer trying to save other reefer money. If his LEDs are above 80lpw then they should be plenty good for MH replacement fixtures. MH is anywhere from 60-80lpw in my research into efficient lighting.
Honestly, even if a given LED has less efficiency than a given MH or T5 lamp, it still may be "better" over a fish tank because it is inherently more directional. With T5 and MH, a certain (significant) portion of the light will pretty much always be lost, given the typically used reflectors (even the good ones). Whereas, with LEDs, given typical build techniques, it's much easier to ensure that a larger percentage of the light is usable in your fish tank.
That said, using lumens/watt to compare an HP LED with an MH lamp is pretty meaningless, since the spectra will almost certainly be vastly different (even if the color appearance is the same to the human eye), and lumens is a unit that is highly dependent on spectrum. Hence, IMHO, lumens/watt is only really useful when comparing light sources with very similar spectra.
In reality, this whole thread is pretty much a meaningless argument, because
we have no data. Surely a fixture can be built with these LEDs that will sustain beautiful corals - no one can deny that. I'm sure many people have done so, and I'm not trying to say that they are wrong or cast doubt on their results.
But, you can also build a fixture with normal output fluorescent lamps that will sustain beautiful corals! That doesn't mean it would be a "good" thing to do according to the typical reefers' selection criteria today. In the absence of data, IMHO, it's prudent and reasonable to assume the given product falls in line with the market as a whole, and given reasonable criteria, there's no such thing as a $2 LED that performs as well as a $7 LED. Going back to the car analogy, there's nothing wrong with a Toyota, or even a Lexus, but surely you can't compare either to a Ferrari. And, given the lack of data, we have no way of knowing if these LEDs are Lexuses or rusty old Kias. I'll repeat it for the third time - I'm not trying to bash, or even come across as negative, I'm just stating facts.