RSI of 90 is relatively rare .... less than 1 occurrence per day excluding repeats so far this year. (Excluding pinks and stocks with less than $100k avg daily volume). And we're in a bull market. Probably much rarer in a flat or down market.
Just skimming over the associated stock charts from the day the stock first hit RSI 90, performance varied but on average seems to be about flat for the next few days and weeks. Some dipped, but others did not. Check out the charts to see the detail.
A few examples, starting on 1/2/13 - (date shown is the first day stock closed with RSI > 90)
1/2 NMR $6.12, closed today at $5.38
1/2 PROV $18.14, now $17.72
1/7 MDCI $4.74, now $4.63
1/8 IEP $51.46, now $63.71 - a solid gainer !
1/10 PRCP $7.47, now $7.68
1/11 RSE $18.17, now $18.47
1/16 PRIM $17.20, now $19.00
1/17 IRE $9.11, now $8.98
1/22 GV $3.40, now $2.79