Well, if you're planning on getting an 06 WRX then a good amount of abuse in any form can cause a gear failure. It won't happen the first time you do it and it could just happen on a hard highway 3rd gear pull. Either you can factor it into your build budget or just go with a STI or 3rd Gen WRX. You could get lucky and never destroy the transmission on a older WRX...but rarely can people hold themselves back from being abusive enough with more power to not break it eventually.
378awhp/373wtq Custom Dom 3 & CBRD tuned 2006 STI.
"When in doubt...flat out" - Colin McRae