I don't think you're ever going to get a black and white answer to your question. It's very hard to evaluate the condition of your suspension this way. Deterioration takes place gradually over time and you slowly get used to softer and softer shocks and struts until one day you realize the cars bottoming out going over a road dip and it never did it before. 35-40 K miles is low for strut replacement but as you said it depends on your driving style, type of roads traveled and road conditions and also the brand of strut. You'll certainly notice an improvement if they are replaced but how much is very hard to say. Excessive dive can also be caused by weakened springs.