Yep, this pretty much echoes what I said above...
Everything else being equal, improving your serve and your return of serve could move you up a half point (3.5 to 4.0, for example), I guess.
These really are important strokes, because they set the stage for the rest of the point. Here's an example:
There's a woman I play against. When we play groundstroke games to 11 (start with groundstrokes, can't lose the point until the third hit), we pretty much split, and the games are always close.
When we play actual sets, I generally win convincingly. The difference is the serve and return. My serve is good enough to put her on the defensive, and my returns do the same. I get to start almost every point at the advantage, and the final game score reflects it.
...I think the point I would make at this juncture is to say that you have to have a balanced game, sure. But a big improvement in serve and return can lead to big improvements in your game...which will translate, eventually, into higher NTRP ratings or whatever. Per what volleyman says, I see players who work endlessly on their groundstrokes, can rally people into the ground, or hit winners, or create unforced errors...in drills that start with a courtesy stroke. Then they get into a match and get shelled and can't figure out why. If your first serve isn't forcing, your opponent probably isn't going to do you the favor of hitting back a courtesy stroke for a return, so you'll likely be chasing balls and trying to play defense as opposed to controlling the play with your groundstrokes. Same is true of the return. If your opponent has a great serve, then, yeah, he's going to get some winners and aces and you will likely start the point on the defensive. But if you get a look at a weak serve and just rally it back, then you just gave up your opportunity to control the point from the outset.
The subject we're discussing...Does serve improvement make a difference in NTRP level? is one of those discussions where the only reasonable answer is "It should, on paper, but once you get on the court, it depends..." What I'd urge everybody at a sub 4.0 level (or any level, for that matter) to do is invest some time in improving your serve and return and see what it buys you. If you take a look at the NTRP level descriptions:
http://www.olypen.com/peninsulatennisclub/NTRP.htm
They don't really talk specifically about the serve until post 4.0, and sort of imply one or two things, or both: (1) You don't have to worry about a better, more powerful serve until you get to 4.5, (2) A better, more powerful serve just sort of automagically happens, just appears, as long as you work on the other parts of your game and get match tough on your path to 4.5.
I look at it the other way, which is if you want to either compete better at the 3.0/3.5 level and/or you aspire to 4.0 and above, you need to start thinking about better serves and better returns while you're a 3.0/3.5. If this sounds unrealistic, I don't think it is at all. It depends on your approach. What I would generally do as a coach, at any level, but particularly at the 3.0/3.5 level, is, yeah, let's work on groundstrokes a bunch, because you have to be solid enough to rally. But once I'm convinced that you can move well enough and hit consistently enough to stay in an 8 ball baseline rally, we're going to take a side detour and work on what I consider to be the two cornerstone shots in tennis, which are, the overhead and the volley, specifically the backhand volley.
A lot of players can hit an overhead, but they can't hit it cleanly, and they
definitely can't hit it cleanly unless somebody lofts the ball right into their hitting zone. So we do overhead drills, until you have that down, and I mean clean stroke mechanics, hitting with power, and making 90% of the balls I throw up at you. Then, figuring out a serve is easy, because
you're tossing the ball, instead of me.
The volley has gone into grave disuse, IMHO, in today's tennis, and I think it is largely because there is a whole segment of the pro game, and I'm thinking mostly of the men, but not exclusively, where the whole idea is "Big first serve, big forehand, that is all you need to know." Yeah, I know it works, sort of, but the guys who distance themselves, and win the majors have what we used to refer to as "an all court game."
I just watched some of the NCAA 2009 tournament, and I was appalled at how limited the game was for players of this level. In the commentary, the strength of almost every single men's player was "forehand", and the doubles was pure one up/one back groundstrokes only...basically, junior high school tennis. Yep, they were NCAA champions, but IMHO, that's all they're ever gonna be.
Enter the backhand volley. Once you get that one down, and a decent forehand volley to go with it, you have a lot of stuff that the heavy topspin only crowd doesn't have. As in, a slice backhand, which is a great, underused weapon. It's all Ken Rosewall ever had on that side, and he did okay, from what I remember. You also have a great short backswing feel that you can translate to groundstrokes, as in when you're rushed or have to return a monster serve.
So there it is. If you're wondering if a bigger serve...and return...can up your NTRP number, I
know it can...but
you have to prove that to yourself. Why not give it a try?