My gutters are 170mm, 1/24 dtm cars can still catch the fence prior to a deslot, and brm Camaros and mustangs also get very close when overcooked. For what it's worth i consider your 150mm gutter a pretty decent number, i have found if the outside wall is too close it gets used for advantage by the outer lane.
Google image Melbourne Model Raceway. This track is 6m long and width would be in the ballpark, i must admit i don't remember the exact width. 5 lanes, 100mm spacing, wide gutters, must be at least 200mm. The track is simple enough to learn however mastery takes a considerable number of laps. The track is well suited to both 1/32 and 1/24 classes, although the brm group c cars were too wide to race cleanly. This may be worth a look.