To answer your questions in reverse order:
In terms of mods, I had upgraded the brakes with cross-drilled/slotted rotors and Hawk HPS pads, upgraded the suspension with KSport Kontrol Pro coilovers, fitted a CorkSport short-throw shift plate and rear torsion bar, and did various cosmetic upgrades (tail lights, LED exterior/interior lights).
The clutch in the new 3 is not nearly as touchy as the one in the 2. It's super light, and the 1st to 2nd shift is a bit juddery at first (until you get used to it), but it feels a lot smoother and you don't have the issue of missing gears because of a sloppy engine mount like you have in the 2. The throw action for the shifter is a bit on the long side, but CS does make a short-shift plate for the new 3 that should fix that, and I suspect that's meant for people who can't manage a shorter throw. Like I said, 1st to 2nd is a bit tricky at first, but I don't miss 2nd to 3rd in this car at all (and I did a lot in the 2).
The power looks underwhelming on paper, but I'm surprised how meaty the 2.0L SkyActiv engine actually feels. The 2.0L I have has 55 more horsepower and about 50 lb/ft. of torque more than the 1.5L engine in the 2. Do keep in mind that you have to offset that power with the added weight, but the 3 sedan weighs about 2850 lbs. (compared to ~2310 with the 2). It goes from 0-60 about 2-2.5 seconds faster than the 2, and I have no trouble overtaking in this car (3rd gear is best at highway speeds...you can go from 60 to about 95 in 3rd gear in this car...compared to 60-80 in the 2). I feel there's a lot more mid-range clout on the highways, so I don't have as many butt-clenching moments when I go to pass someone.
Even though you didn't ask, the build quality for the new 3 is phenomenal for what it is - an affordable compact. It feels a lot nicer than the last-gen 3, but it still has the enthusiastic and well-composed handling of its predecessor. The road manners of the car are exceptional, coming from the 2. The ride is informatively stiff, but by no means uncomfortable. However, like other Mazdas, to get decent handling you have to sacrifice a bit in the noise department. The road/tire noise isn't excessive, but it is a bit noticeable at speed. Way quieter than the 2, though.
To sum up, the new 3 is one hell of a good car. It's reasonably comfortable and quiet, the 2.0L SkyActiv engine is super smooth and quiet, if not as powerful as, say, a Volkswagen (like I said, it's still got enough grunt to make you happy coming from the 2, and it does sound nice). The clutch action is light, but not as lacking in feel as the one in the 2 was, and the transmission itself is nice, especially since it's a 6-speed.
As a side note, I did have to teach myself how to hit 6th gear in this car...With the longer throw, you have to push outward (to the right) a bit to get from 5th to 6th. When I went on my test drive, every time I went for 6th I kept getting 4th instead.
Oh, and if you didn't know this already, to engage reverse you have to push down on the shift lever, then move it left and forward.
Hope my insight helps you with your decision.
