I'd send him a message requesting info on what he actually does to that particular mic. $100 is a bit steep if all he's doing is swapping out a few caps. If you aren't happy with your 990, you're probably better off selling it and putting the proceeds +$100 towards something like an Audio Technica AT2035. You could also spend $20 on an eBay mod kit and a Radio Shack soldering iron and attempt modding it yourself. Don't expect a profound improvement, though. I performed the "Gus Mod" on a pair of MXL 603S mics (same circuit and capsule as the 990) and they do sound better to my ears. Here's a link to some instructions. Gus' MXL 603s mod - Pictures/Part numbers -thanks Flatpicker
its $300 replaces the diaphragm with a k47 like diaphragm, modded electronics, and basket. sounds very much like a u87, nice.
been thinking of getting one he has already modded. since he did a great job with the one I have.
the KM84 variant I've head good things and bad. On some applications, the pattern is a little bit tighter (more hyper-cardiod) when compared to the KM84, but sound more like the original than the KM184. I am planning to buy these eventually. But I have a set of KM84's at my disposal, so it is not my top priority.
How much are you into DIY? The 990 has the same circuit as a million other Chinese mics. I modded a 2006 (same circuit) with an OEM C12'sh capsule and directly connecting the capsule to the FET. I didn't do any other upgrades, but if you scour some of my posts here, I am pretty sure I posted a 990 specific mod.
Michael Joly is a magician and I'd also recommend this route if it's in your budget. He modded my Oktava MK319 and I absolutely love it.
...all of these recommendations aside, why are you looking into mod your mic in the first place? I ask because depending on your recording environment and the rest of your equipment, the mic may not need to be one of your top concerns.