If you make it up to 23L it will be correspondingly weaker in alcohol and possibly taste a bit thinner.
The AIPA has a lot of dextrose in the kit IIRC which bumps up the alcohol content without really adding to the body of the beer. I imagine that adding extra sugar to bump a porter up so that it is the same ABV but 40 pints rather than 30 pints mgiht result in a beer thats a bit thin for the style.