The notchiness/grinding has always been present in the integra line... It has to do with the synchromeshes wearing, and the percentage of wear leads to a higher percentage of RPM-discrepancy between the dogs' teeth and the gears. It's not noticed in 3rd thru 6th because the percentage of discrepancy is significantly lower. It's not seen in 1st gear, because the honda trannies have a super-paranoid synchro mechanism on 1st gear that doesn't let you shift into it unless the engine and driveshaft are dead-even on speed (or you're stopped). If you want to test it, roll up your windows, turn off the air and stereo, and get up to 15mph in 2nd gear. then patiently downshift to 1st, holding the stick against the gate into 1st. You'll actually hear the layshaft in the tranny spin up before it'll let you in. Noted, it's not a good idea to enter 1st gear at anything above 5mph on our cars, so don't yell at me if you break your tranny!
On my teg, I've downshifted from 5th to 2nd numerous times (of course, never when I'm above-redline for 2nd gear), always by double-clutching and rev-matching the engine. When doing this, I have no problem with engagement or grinding and everything goes great.
Note: I have screwed up a downshift to 4th when doing about 80... I caught the dead-space between 4th and reverse while doing a hard shift... It totally ruined the synchros for 4th, 5th, reverse. That was replaced on warranty because the problem had been seen many times by the dealer. Apparently it's a quirk in the lever-arm actuation that causes the control arm for both the 3rd/4th and the 5th/reverse in B-series trannies to engage at the same time, fighting each other for synchronization.
For those poor dummies who blew their RSX's engines and trannies by missing downshift from 5th to 4th at above 85mph (65 is about redline limit, but 85 is roughly the blow-up limit), they just need to understand their car's gearing and the fundamentals of manual transmissions better. Any reasonably-understandable shifting mistake (like downshifting from 5th to 2nd at 75mph) will be properly handled by the rev-limiter and a mild dose of press-the-clutch-cause-something's-wrong intuition. If you do something that could only blow up your car at 15mph over the highest speed limit in the nation, they have a strong common-sense case to deny coverage based on abuse.
If you want to know manual transmissions, synchros, and other engine basics a little better, check out the tutorials on
http://www.howstuffworks.com/ ... They're quite simple and easy to understand.