The storm collar on steep slope roof will work very well, if the Retro-Collar is purchased for the size of the conduit/pipe diameter, and the collar is pushed down over the existing flashing opening.
However, if the old flashing is a real messed up combo flashing that is made from neoprene or silicone and the rise of the metal part of the flashing is very low, I would highly suggest replacing the entire flashing.
It is much more secure if the old low profile flashing is cut out, and replaced with a new flashing with at least a 3" rise above the roof deck.
The roof repair on this type of flashing repair is quite easy.
I always used to spud back the gravel, prime the area, install the flashings with nails, and then use an SBS sheet tied into the roof with a good overlap of at least 6 to 12 inches. Set the SBS modified sheet into a bead of good quality wet patch plastic cement. I used wet patch only because the standard plastic cement is harder to spread, and IÃ¢â‚¬â„¢m a lazy guy. To make the repair look good just cover the SBS patch with gravel/rock. In a day or so you cannot take the patch off, and it will never leak.
Torch down can be used also, if your crew is good with a torch. IÃ¢â‚¬â„¢m always nervous around flame throwers on any roof job though, especially after my crew almost burnt a famous singerÃ¢â‚¬â„¢s house down doing a small repair. It was lucky I was near by with a hose.
Our spin flashing design will do the trick as a good replacement flashing, and the Retro-Collar will counter flash the flashing. The work will look professional, and a roofing contractor can make a good buck doing the work.
Let me know your feelings.