Same performance with no plastic bubble.
Despite being 1400sq', single story, with an abundance of windows, the light brown walls ate all the light coming into our home, making it dark and depressing even in the summer. So, when the time came to re-roof, we installed 7 Velux skylights all together. 2 electric 4646's, 1 fixed 3046, and 4 tubes.
Two of the tubes would be visible from the street, and I am not fond of the appearance of the plastic dome. Luckily I discovered the TLR low-profile unit while planning the project. Another
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reason for choosing the TLR was the pitch of our roof was such that a TMR, which our roofer recommended over the TGR for our roof, would have sat askew/out of level. We did go with the TGR instead of the TMR for the domed units after meeting with the Velux rep, but that's a different story.
****Bottom line: The Velux Sun Tunnel skylights bring in much more light than we were expecting (after seeing our neighbor's new tubular skylight), and there is no discernible difference between the amount of light coming in through the two Sun Tunnel's with the plastic dome, and the two with the flat glass termination. The finished appearance is discreet and attractive, and was worth the extra hundred dollars.
In fact, though I love the two enormous venting skylights in our living area, in hindsight I may have opted instead for 4 TLR tubes, which would have resulted in as much and possibly more light, less cost, and less additional load in heating and A/C.