Source Four Lens Tubes: 36 --> 50 seem like a big jump?

The Jr definitely won't help you in this situation since you are lamping at 750 (Jrs are 575 only).  But, I am curious to see what using an EDLT 50deg does to your brightness issue.  The increased light output may be enough to make up for the shuttering.  

Well... if you remove the screw (iirc), the 750W lamp works fine in the Juniors... granted I'm sure there are those who will say this ruins the reflector, shutters, lenses, etc... but anecdotally, we've never had an issue with doing it.

Yes, we should probably try the EDLT tubes, although again, there is the added cost.

Modifying the lamp base to accept a 750w lamp in a S4 Jr fixture is not a recommended practice. It would at the very least invalidate the terms of the regulatory agency marking on the product.  Degradation of the components due to the additinal heat generated may not be readily apparent and may create undetected safety or liability issues down the road.

The same thing was said about the original 575W-only S4.  Then we were told by engineering that replacing the cap with a 750W cap was actually entirely fine for the reflector etc, and then that actually the original cap could accomodate the 750W lamp in the first place...  So, yes, I understand the caution, but am somewhat skeptical.

The problem with your assertion that if it is okay with a full size fixture it will be okay with the Jr,  is that a full size SourceFour and a SourceFour Jr are very different fixtures. 

There are two critical areas to be considered here.

1.)  The reflector is a smaller size  than a full size S4 reflector, and is therefore also physically closer to the lamp. 

2.)  The cooling chamber surrounding the lamp / reflector area in a Jr is significantly smaller when compared to a full size S4.

Both of these factors together combine to create an increase in the operating temperature of the fixture. Furthermore, the fixture cannot properly dissipate the additonal heat created, which can cause a variety of heat related issues with the fixture long term.   Paint issues, cracking reflector, lamp life, fixture temperature and so forth.

if it were considered to be safe and proper operation to lamp a Source 4 Junior with a 750w lamp,  it would be marketed as having this capability.  Unfortunately the Source4 Junior is one of two fixtures in the entire Source 4 Line that cannot be rated for use with a 750w lamp. The other fixture being a Source4 MCM Par. Both of these fixtures should not be lamped higher than a 575w lamp for safe and proper operation. 

Making this modification is strongly advised against by ETC, and would invalidate any warranties or UL / ETL / CE regulatory approval ratings assigned to the fixtures.

The problem with your assertion that if it is okay with a full size fixture it will be okay with the Jr,  is that a full size SourceFour and a SourceFour Jr are very different fixtures. 

There are two critical areas to be considered here.

1.)  The reflector is a smaller size  than a full size S4 reflector, and is therefore also physically closer to the lamp. 

2.)  The cooling chamber surrounding the lamp / reflector area in a Jr is significantly smaller when compared to a full size S4.

Both of these factors together combine to create an increase in the operating temperature of the fixture. Furthermore, the fixture cannot properly dissipate the additonal heat created, which can cause a variety of heat related issues with the fixture long term.   Paint issues, cracking reflector, lamp life, fixture temperature and so forth.

if it were considered to be safe and proper operation to lamp a Source 4 Junior with a 750w lamp,  it would be marketed as having this capability.  Unfortunately the Source4 Junior is one of two fixtures in the entire Source 4 Line that cannot be rated for use with a 750w lamp. The other fixture being a Source4 MCM Par. Both of these fixtures should not be lamped higher than a 575w lamp for safe and proper operation. 

Making this modification is strongly advised against by ETC, and would invalidate any warranties or UL / ETL / CE regulatory approval ratings assigned to the fixtures.

I'm not actually asserting that one should do this; just saying that it isn't immediately catastrophic.  However, we're quite far afield from my original query...

I'm not sure where you are located, but if you are able to contact a local ETC Dealer or Rep Agency they might be able to get you a loaner of some 50deg EDLTs to try out.  

Even if ETC decided today that they were going to make a 40ish degree lens tube it would take some time before one would actually be able to purchase it.

Obviously, there is a cost difference between an EDLT and a standard lens tube.  But, if it solves your problem just as effectively as using 25-50 zooms, then spending a little more for the EDLT (which is significantly less $ than a Zoom) seems like the right move with the current product offering.

The EDLT can give you up to 20% more output from the fixture, see the cut sheets for the EDLT and the Standard 50deg:

http://www.etcconnect.com/docs/docs_downloads/datashts/7060L1042_S4_EDLT_Spec_Sht_vC.pdf

http://www.etcconnect.com/docs/docs_downloads/datashts/7060L1010_Source_Four_50_Spec_Sht_vG.pdf

Personally, I'd go for a 41 degree (mf=0.75) or a 44 degree (mf=0.8)