You must absolutely abide by the speed restrictions whether you're in IMC or VMC. As has already been stated, if you track via an instrument approach at night you are doing an instrument approach until you meet the specified criteria for being able to do a visual approach and therefore, the approach speed restrictions apply. Whether you're in IMC or can see the aerodrome from 50 miles away is of no consequence.
This is poorly understood by many people, including quite a few training captains I've come across. At night, when someone says "Well we're visual, are you happy to keep the speed up?", they're showing a fundamental lack of understanding of the differences between day and night visual approaches. The confusion arises from the difference between VMC and being 'visual'. You're not visual at night until you meet the criteria, no matter what the weather is doing.