Many projectors allow for true 1:1 pixel mapping as one of the input options. This is ideal for an external scaler as it won't screw up the image a whole lot on the pass through the projector's processing. Most projectors STILL will apply some bit of processing, but this may just be for color, not for actual image manipulation.
As far as your DLP vs. LCD vs. LCoS question...
I'm a big fan of DLP, but I don't own it. I have installed many different projectors and I consistently see results from DLP that are just a touch better than LCD. Yet, I'm not entirely unconvinced that LCoS, such as the new JVC, can't best, or match, DLP for similar, or less money.
Recently though, I installed the BenQ W10000 projector, a 1080p DLP model, with lens shift showing on a 106" diagonal screen. Gotta say, it was really incredible.
One of the hardest parts of showroom demos is that you could be seeing a lamp that has 1,000+ hours on it and it may not actually be calibrated correctly. Both of these things will greatly reduce the overall impact you will get, but the lamp hours is a good indicator of how many projectors may look after you put a bunch of hours on it.
I remember when my projector began to burn it's lamp out - it was very dim, with low contrast, no punch at all, and basically sucked. When the lamp blew and I replaced it, I felt like I was back at day one! It was absolutely incredible the level of difference.
Now, is DLP a ton better than LCD or LCoS? Not at all. It is a half step better - at most. If DLP is a A-, then LCoS is a B+ and LCD may be a B. CRT, for pure image quality is still the A+ gold standard... when properly calibrated (almost never).
The biggest issue I have with the Optoma models has got to be the fixed lens (no lens shift) and the horredous lens offset which basically required 9 foot+ ceilings for proper screen placement. Also, I seem to recall that the throw distance was a little bit longer than it should have been for a projector with 1080p resolution. This is not an issue you will find in projectors like the JVC, Panasonic, or Sanyo 1080p projection models.