They wont be as high quality as the original 907 handpieces. People have brought them in the past and found them wired incorrectly, so if you go down that path double check before using and don't blindly trust it. As for that particular handpiece noone can determine the quality with certainty, it's an unlicensed knockoff (aka outright counterfeit).
The problem with the finer handpieces and tips is the mass and power delivery difference due to the smaller size. It's not going to be better at soldering but it might heat a little faster, overall performance though will be lower, it will struggle to do everything the 907 can. That being said someone is selling a cheap used 900S on ebay for about $50 with shipping. But really you are not gaining anything so I wouldn't bother.
I would buy a genuine handpiece if it was me, but I think a better idea is if you have the cash and doing lots of soldering ditching the Hakko 936 for a new Pace ADS200 soldering station, it's only ~$220 with discount delivered for the instant setback version. It's cheap to run, fast and has great performance. Then you could either slap a bandage on the 936, keep it as a second or sell it to partially offset the cost.