The main downside to shortening the fluid change intervals in your transmission is the increased cost. The benefit is obviously prolonging the life of your transmission. If you have a way to raise the front end of the car (jack + stands, ramps, lift, etc.), some very basic hand tools (plus maybe a T-70 Torx bit, depending on the year of your Subaru), and a little time, you can change the transmission fluid for a fairly reasonable price. The used trans fluid can be mixed with used motor oil and recycled together too.
The last time I changed my transmission fluid with the Andrewtech Cocktail, it cost me:
(1x) 1 Liter Motul Gear 300 75W-90 = $15.95
(3x) 1 Quart Redline Lightweight Shockproof Gear Oil 75W-140 = $10.90/ea. = $32.70
Total = $48.65 (not incuding shipping)
The factory recommends changing the transmission fluid every 30,000 miles: Subaru maintenance schedules and new car break-in period- 2000 through 2009, links for 2010, 2011...
Since I now have aftermarket Albins gears, it was suggested to me by an Albins representative directly as well as by several employees at IAG Performance (the shop that installed the gears) to cut that fluid change interval in half, so I'll be changing my trans fluid every 15,000 miles now. Both also said the Andrewtech Cocktail is a good formula to use.
If you don't know when your transmission fluid was last changed, I highly recommend you change it as soon as possible to help prolong the life of your transmission. Trans rebuilds are not cheap <-- I speak from experience