Ian knows these things, but just to clarify...
A 150 should land on 265 or so when you remove the front cell. If you are trying to use the lens as a longer lens (and for that purpose), you have to use it at small aperture openings (else it's very soft and low-contrast). Also, most convertible Symmars I've seen are normally at f/12 when the front is taken off. So, if your shutter doesn't have the dual aperture scale, try calculating with 2 1/3 extra stop for starters (set the aperture at f/22 and it's really f/45 1/3).
A yellow filter (orange, red ...) will help both sharpness and contrast, as the results to start with arn't that impressive.
If you really want a longer lens, this is a "last rescue". The only way to get quality is to buy a longer lens.
//Björn