Roughly how much hp gain is this set up meant to give out of interest?

Technically and officially - none. Although there will be some increase. But I don't do rolling roads (well not until I get a remap

), so I can't actually tell you how much gain it is giving. But it is a definate improvement.
The main advantage of bigger and/or additional intercoolers is a considerably more stable output even under very high ambient and underbonnet temperatures. These really come into their own if you have other tuning work done as well, from a simple remap, through to zorsts, hi-flow sports cats, bigger turbos, uprated injectors and fuel pumps, etc.