First there was Q4s with more than 300hp and should be some now also I guess... You can find some in old threads on here, but unfortunately old garage feature of the forum with photos and specs is now gone and most of the pictures on old threads probably are not available anymore...
Yes, for sure there is much more powerful Integrales than Q4s and in my opinion the reason lies on the same things which made one car (Integrale) much more popular than other (Q4) - produced numbers, more special design and famous racing heritage association. There is much less Q4 produced (because wasn't high demand probably) and the model look to me as always been in shadow and not well known even when new. Also Q4 look much like a normal car unlike Integrale especially in Evo form, which add to its inconspicuously. They have to made it a little more distinguishable in my opinion... The Q4 is homologation special for racing car also, which was very successful and that is GTA, but that fact is like a secret and also GTA short lived. Will stop here as this is another topic .
Mechanically I don't see why to not be able to achieve similar power as main components are similar - engine, transmission and drivetrain. Integrales are lighter so probably a little bit less load on components.
Do nоt fully believe on advertised power figures from sources around internet, not everything, which you read is true. For off the shelf chip my approach is to look at what offer reputable companies specialized in particular makes. For example Squadra Tuning is specialized in ECU tuning of italian cars from many years and is well known with good feedback. I see in their site to offer 235hp and 345Nm with requirement to be used with 98RON fuel and that sound correct to me for standard car -> link
295hp is not possible on that model without modifications and I don't mean only bigger exhaust and air filter.
For comparison with Integrales of modified chips have to be mentioned that even the smallest turbo used in Integrale in Evo II and which is similar to the one used on Q4 (TB0367) is not exactly the same and have bigger power potential from what I have read (bigger compressor wheel and turbine housing). Integrale 16V and Evo I use bigger turbo and on 8V is bigger than these.
From all versions of Sierra and Escort Cosworth only the smallest turbo used on latest Escort Cosworth (T25) have power potential around Q4 turbo, but from looking that it make close to 300 it maybe flow more. All others are bigger and biggest used on Escort Cosworth T3/T04 with 0.63A/R can make 400hp+.
So Q4 don't have big power potential in standard form with standard turbo in the first place.
A lot of modifications require custom ECU tuning. On www.lanciahf.eu site there is some stage chips.