Well the most recent version of their tiny (2.0) engine is making 306 hp, so I'd rather use that as a real-life example of what might be possible... One could decide to "aim" as high as, say, 500 hp from a 1.5 engine, but there's no data supporting that possibility -- for a stock/production engine anyway
Keep in mind that the CTR's 2.0 has a different reliability target and true VTEC - something the Si will not get. If you want to compare a more mainstream 2.0, look at the Avancier's 265 HP variant.
That said, this car's 1.5 has been reported to get up to 230 HP in testing. As I mentioned in another thread, based on the current Civic lineup and competitive environment, the Si should land right around 215-225. That is, if they can extract those numbers without sacrificing their long term reliability goal.
Something I don't think I've ever seen mentioned is the possibility of the Si using the Sport 1.5T engine with an "overboost" feature? Use the base 180hp 177ft.lbs. but with the overboost feature increase the hp to 205 and torque to 192 by increasing the boost for a short amount of time.
Other manufacturers seem to be doing that and seems like a possibility here considering the nature of the Si.
I had friends with the old 8th gen 2.0 who were running 500hp boosted daily on stock internals with no issues. Just imagine what the CTR motor will be capable of.