You happen to be in the same quandary that I am in. I have leased my MY09XT and I love it but want to get more out of it. How much, well I'd like to be above 300HP and Torque, have an awesome suspension set-up wheels tires, the works. Is this achievable for me. I have pondered this for a while, I have the FOZ for a bit over 1 year now. The answer is NO.....well for me. While I have done some mods (very minor), you can see them here...
http://www.subaruforester.org/vbulletin/f115/hot09xt-journal-55428/, they are the type I can live with if I have to give them up. You know when the lease is up. I may buy the FOZ at the end, let it go and get another, or get something else. I have owned sooo many vehicles now I really don't get too attached any more, when I was younger yeah but things change and vehicles are changing even faster than before IMO. If that weren't the case then I would not be in this vehicle now. I will say that I really do love my FOZZIE. I am very impressed. For the $$$ I spent and what I have gotten and what the vehicle is capable of, it a winner for me. Back to mods........I will probably do F&R sway bars with end-links. Maybe some other minor stuff that can be swapped to another or sold if I make a move from Subaru. Regarding warranty, as said, it depends on your dealer, mine is great and the guys there are modding people, they race, sponsor races/events so IMO they are more apt to help me out. That being said there is the issue about not using SOA parts that have been designed by Subaru for the MY09+ Forester. Subaru just hasn't come out yet with them. Subaru is very very, did I say very, slow to act on this though they eventually do. This is how I see it form my research, some may disagree but look at what they have for the euro and especially JDM markets. So to conclude I would say "if money is not an issue and you want to turn faster than an STI for cheap, go for it". For a short lease I don't think, generally speaking, much will fail that you will have to worry about. Just maintain it. Subbies when maintained can be and are very durable. Just don't get too extreme.
Oh, I forgot to mention..........the other reason I haven't done much modding (engine wise) is that there was a "stop sale" issued on the MY09XT. Some have slipped thru Subaru's rigorous testing :raspberry:. Of those, some have failed early in life and some later, so who knows if mine will. This is just something I am living with.
I Hope this reply helps you somewhat. On a lease I wouldn't get too crazy unless you are leasing thinking you will not like the vehicle and can walk away if so. I think you may, like most new to Subaru (present company included) that I have seen here, really like/enjoy the Forester.......the Turbo more so, except for the engine failure.
I have only seen an occasional......"I don't like this vehicle".