That's a good feature request. Premiere Pro is hard-wired to double the playback speed each time that we press L, but it would be great to set 1.25, 1.5. 1.75, etc.
You can increase the speed of the clip a little bit such that it plays faster than 100% playback speed (Clip > Speed/Duration), but I'm not sure that's a good way to approach it. It might work for setting Markers and changing it back to 100% when done.