The simple solution is to fit a manual boost controller and why anyone would change to a bigger intercooler and not do that is beyond me.
Reason why it's down on power is to do with the volume of air being shifted.
As the intercooler is now larger its able to hold more air per square unit which means instead of shifting say 50 units of air, its now capable of shifting 100 units of air, but the car was optimised for 50, making it feel half as effective.
Essentially its like fitting a larger sink in your kitchen to the same sized tap. The water can't flow any faster, and actually takes longer to fill up unless you change to a bigger tap.
You find typically on our cars a larger intercooler will drop 4 or 5 psi. This doesn't mean the cars not right, your just gonna have to adjust the boost representive to new parts and air moving capacity.
Pressure per square inch here folks. You have fitted a bigger capacity unit and need to adjust the pressure intake accordingly, the psi setting was based in your old factory components.
Obviously you can then bring it back to factory 17/19 psi using a manual boost controller or up to 23/24 psi before boost cut kicks in.
I've mine all set at 22.5 personally along with mafless remaps to match, egr bypass and decats, and barely a drop of smoke on full throttle as they are mapped correctly. With plans for solid steel hard pipe work replacing everything plastic next and dual maps once I've my white 75 mot'd again.
Only caution I'd say about setting the boost is there are boost controllers specifically for diesels, I'd highly recemmend you get one for none vnt turbos and one with a quality locking grub screw that once set won't vibrate loose to over or under boost again once set correctly.
Even a decent cheap unit will set you back £20, takes 10 mins to fit and you read the boost via the odb plug on toaf or any odb car phone app.
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