I had my bike professionally fitted a few months back and the guy made some changes to it. One of them was going from a 100mm stem to a 60mm stem. The change felt better for my back, but the bike feels really hard to control and I can barely ride it while standing. I tried going back to the 100mm and just pushing the saddle forwards to compensate and have been feeling the position to be about as comfortable, but the bike handles well again. The question is: is pushing the saddle forward by more than suggested to me by the fitter (almost all the way) a bad idea? will this have a negative impact?
Assuming the fitter correctly positioned the saddle set back for you, moving the saddle forward could result in more knee stress and the potential for repetitive stress injury. Keep an eye for early signs. Your knees, hips and IT bands could all show signs.
Typically, the recommended order of operation in the fit is to first set the saddle height and setback, then set the handlebar reach. Saddle position is considered one of the most important positions for overall performance.
I find it interesting that the more shorter reach setup relieved back pain. Usually a stretched out position affects the shoulders and neck. You should also look into if you have back and hip flexibility issues. Fit can accommodate flexibility issues, but shouldn't be a substitute for addressing the underlying issues as they likely affect other aspects of one's life.
This is rampant speculation (take with a grain of salt), but do you sit a lot? If you slouch a lot while sitting you may have impacted your back and hip flexibility, giving you a more rounded back riding position and a shorter reach.
As a final aside if you carry a load on the bike, shifting the pacement of the load ahead of the handle bars can return some of the handling characteristics.
There are many factors associated with bike fitting, not just the reach to the bars from the saddle. It seems that for optimal fit your bike frame is probably too big, you will have to compromise on ideal setup. But set up is always a compromise, as no two bodies are the same (and you don't have a custom bike). I am assuming this is a road bike and not a mountain bike.
Physics controls the the way the front end feels - it is dependent on the size of the wheel and the length of the lever controlling deflection (this is very generalized). Bigger wheels are harder to deflect. Longer levers make it easier to deflect, as you have more...leverage :). There are way too many factors to go into, but the position of your hands on the bars, the stem length, the width and shape of the bar, and the position of the controls all come together to create that lever that steers your front wheel. A shorter stem makes it twitchy, 90mm is pretty neutral, and the professional sprinters, like in the Tour de France, often have 140mm stems, for stability in the tight finishes. On mountain bikes, a narrow flat bar is more twitchy than a massive 760mm wide downhill riser bar - the lever is longer from the center control point.
As for seat setback...that can impact your power, it's not just about how far you are away from the bars. You can test it out by pedaling hard on a small rise - push yourself back on the saddle for one test, ride in the middle (where you normally would sit on the saddle) and then make a third attempt riding more forward, on the nose. You should be able to feel when your power is engaged with the pedals - that would be the ideal point for your saddle - and when it isn't, and it feels like a slog. This is also physics and the lever in this case is the femur. Bike fitters often use the marker of a vertical plumb line that drops from the knee to the pedal axle when the pedals are in line parallel to the ground. This, of course, varies - positional power also depends on your muscle power (if you have stronger quads vs stronger gluteals).
As for fitting your bike - you will probably need to balance stem length with saddle setback and find something in the middle - pull the seat forward a bit and use a 75mm stem with a decent rise. You need to balance your reach to the bars with your position over the bottom bracket.
Also remember saddle height - the saddle comes up at an angle when you raise or lower it (the angle of the seat post) so your distance to the pedals and the bars changes as you raise and lower the seat. If you move your saddle forward and back, you will need to raise or lower the saddle accordingly, until you get a good fit. Good fit means your hips are stable and level and don't rock - one way to adjust this it that when your pedal is at it's lowest point, with your heel on the pedal, your leg should be fully extended - not bent, not hyper extended and your hips shouldn't rotate so your heel can reach).
Experimentation is the key. A good bike fitter can get you to an ideal, but it may not be what works for you, everyone is different and we all have different riding positions depending on what we want to accomplish - like beating your buddy to that next sign post or riding 200 miles in a day. If you aren't comfortable, you won't ride....and that is no fun.