Just a quick question: What are you using to measure the fL as that seems pretty low compared to what I'm getting from an X35. My screen is narrower at 112" but the same gain at 1.5. I'm using minimum zoom and getting 14fL when using my Isco II lens and -9 on the iris in low power. I found that the fL readings using my i1 display Pro enhanced meter were low compared to my calculated fL based on a lux meter reading at the screen. This is due to the directional nature of the screen making it hard to get an accurate light level reading with the i1D3.
I found that there wasn't much difference between zooming brightness and using my lens either. This is likely because I'm at minimum zoom and there is a big change in brightness as the projector is zoomed at this end. There is likely a big change in contrast too, hence why I keep the lens. I wouldn't base getting a lens on expecting a huge difference in light output due to the effect of the projector's aperture altering the light output as it zooms. When I checked with my old HD350 it was something like 78 lux (lens) verses 74 lux (zoomed), so not even worth worrying about as you'd never see that difference even side by side IMHO.
The throw is going to be around 2.0 using these figures, so pincushion will be worse than I suffer (negligable) at 2.8 but well above the usual target of 1.5+
138" wide 2.35:1 is 103" width without the lens. 17' throw is 204" so I make that 204/103= 1.98 throw.
If it wasn't for the noise issue, I'd say just stick without the lens and buy a lamp when it dims too much, or do the same in low lamp and just accept a dimmer image. I meant to ask, but I'm presuming that you've opened up the iris (aperture) as well?
Zooming: Been there, done that, bought the lens...