Ben I did a quick test with you in mind last night. Bear in mind I don't do visual, I'm an imager. I'm getting larger image scale with my Meade APO 2X barlow + extension tube than with my Televue 3X barlow. I can do some measurements (from 2X barlow elements to camera sensor) If you'd like, but I'd doubt the extension tube itself is much over 100mm in length.
This was with the 11" SCT on jupiter. Either the math is wrong or I'm not actually getting 3X the image scale with the 3X barlow
