Well, there are more than a few people running 2.5 engines off the 1.8 ecu. And with the exception of a few people that got bad motors, they seem to be having pretty good luck. I agree that it is by no means ideal, but compare the price of a 2.5 swap to the price of a WRX swap. And then add on all the work that you have to do to get it (WRX) in there. There really is no cheaper and easier way to increase HP. For less than $800, you can get 165 HP. For approx. $3500-4000, you can get 227 out of a WRX. So, if one does the 2.5 swap, one would save on average about $2500-ish. Take that money, toss in a set of cams, equal length headers, maybe use a set of 2.2 heads, thin headgasket, and you're at about 200 HP. And you'll still have money left over.....
If one has the money and time to spend on a WRX swap, then I say go for it. I might find myself in that situation this fall.... But, if your on a budget, and your new to swaps, then the 2.5 is a decent place to start.