Hard to say what kind of difference it will make for you. Lots of variables go into the perceived feel of how a bike ‘fits’ each rider.
My experience: I shortened my stem and at the same time put on wider bars with 15 mm of rise. Felt like riding a brand new bike. For me it was exactly what I needed to make the bike fit me and how I ride. Hope you have the same experience.
Like Noggus mentioned, it’s hard to say. In my experience, small changes do make subtle differences that sometimes turn out to be more impacting than first thought. But regarding your goal, getting more lift on the front end… I actually gained unintended front end lift by sliding my saddle further back 15mm while adjusting to align my knees/pedals. etc. If you think about it, makes sense. You’re moving your core weight further back by moving the saddle back. I know overall bike geometry (seat tube angle, specifically) plays into it, but you may try that first, assuming you haven’t already. If you have the room to adjust, there’s no part cost, and easy to mess with.
I have played around with this type of thing quite a bit. I have two older MTB’s both of which came with long stems. I have tried all the way down to a 35mm stem on my hardtail and it was great for bunny hopping and riding street. When it came time to sit and spin up a hill however, it was all over the place with the lack of weight on the front end. Ultimately I decided that a more realistic stem length was 70mm stem and wider bars (29″)since the geometry of the bike is more XC oriented anyhow. It struck a nice balance that I can still hop around with but not quite as easily as with the shorter stem.
My full suspension bike was the same story really, I tried a 60mm stem with a bunch of stack and the front would pop up with incredible ease. This was super easy to jump with but I felt cramped and once again I found that the front would wonder all over the place due to lack of weight on the front end. I lowered the stack which helped but I recently moved to a 90mm stem. I have yet to try this set up but I expect a more balanced feel.
Also, the bars on my FS have a lot of pullback or back sweep or whatever it’s called so it makes the reach even shorter.
My advice is to get measured for a bike fit and go with the recommended stem length. You can go slightly shorter with wider bars but going super short will likely have the same effects I noted above. If you are comfortable now, maybe try adding spacers under the stem to give you more stack, this will also aid in getting that front wheel up.
I played around with stems and handlebars for months before I found what ‘fit’ the best for me and my riding style. Here are some tips that I discovered that might help.
When you are riding in a comfortable position, where is your seat bones on the saddle? Front, middle or back? I prefer my seat bones on the back of the saddle. My body structure makes me a bit top heavy so for me this is the best position for front to back balance.
When you are in attack position, is your belly button in front of or behind the bottom bracket? Again, I am a bit top heavy, so I prefer to be slightly behind the bottom bracket.
Does your lower back get sore after riding more that two hours? If so, you may need to be in a more upright riding position meaning, shorter stem, higher stem angle and/or riser bar.
Do you do more XC riding or gravity riding? You should be more upright for gravity riding.
Each bike and rider are different. It is worth exploring what fits you the best for your bike. I can say beyond any doubt that once you find your fit, you will be more comfortable, better balanced and more confident in what you and your bike can do.
How much difference going from a 100mm to 70mm stem will make depends on a bunch of factors that others have gone over above. I would add that if you plan to go to a shorter stem also consider going to a wider bar. I bet that if you have a 100mm stem you are probably running a bar in the neighborhood of 720mm. Any wider and your bike probably already turns like a bus. I would also add that you have to balance bike fit with how much you want to unweight your front end. Current trail and enduro bikes are lengthening the front end of the bike in order to be able to run a very short stem and wide bars and still allow for a correct seat position. If your bike is not designed like this, then whatever you to your stem will be a compromise. That being said, you should be able to shorten the stem 30mm and be able to compensate on the fit. Again, wider bars will uncramp the cockpit allowing for this change. Depending on your size and the size of your bike you might even be able to go shorter than 70mm. If you are unsure, talk to a shop that knows what they are talking about and they can help you get set up the way you like. Yeah, a 100mm stem will put you way over the front end in most cases making it hard to pull up on the front end as well as putting your weight really far forward when you are going down steeps. Good luck.