In your example, "time wasted" = "time which [had been] wasted". It is a reduced relative clause and wasted is a past participle, not an adjective. The tense of the auxiliary verb "to be" depends on the main verb and so I chose "had been" here because you should have said, "had been wasted". When you want to use a participle before a noun as an adjective, you need to check a dictionary to see if it can also be an adjective e.g. "finished", "broken", and "required" are adjectives and so can be placed in either position. However, "compared" is not an adjective so you can't say "a compared item". You would need also to consider the overall sense of the sentence and common usage. Some participles may be used as adjectives in certain collocations but not others e.g. "the built environment" is a known expression where "built" is an adjective. But you would always say "the house (which was) built by my neighbour" ("built" is a participle here).

In my opinion as a non-native speaker, both your sentences are correct and the meaning doesn’t change. But I believe it would be much more common to say “it has been a waste of time” “Would this sentence still be grammatically correct” is correct “Would this sentence be still grammatically correct” is incorrect By the way: You can’t say “or would even change its meaning?” It’s not completely clear what it means and it’s also grammatically incorrect. UPDATE: I agree with Michael Chambers that "wasted" in not an adjective in “time wasted” - I hadn’t thought of that. Whether or not you can use “has been wasted” instead of “had been wasted” depends on which "time point of view" you're speaking from in my opinion.