It's always to get to a good baseline with service parts , I have had this issue before and the boost / map sensor switch solenoid wasn't switching in the black box I would check that for a start 

Any charts for the 2.5L engine with just a tune?

Downpipe and tune is still best value upgrade though it costs more every year.   Stock 2L cars are only really about 185kw at the wheels, with down pipe and tune can hit 220kw at the wheels.   But that’s just peak power which isn’t where the big gains are. What you want is more area under the line on the dyno graph. Useable power is consistent and found in the mid range.   Stock turbo won’t put out much more than a few extra psi at the redline so up there there is only a little gain up there. What it can push 5-7psi more or about 23psi by 4K which is a 30-40% gain. Turbo comes on a little earlier and ramps up way quicker. That’s where the push you back in your seat comes from.  Look at the power on some dyno graphs posted on here or other forum that have stock and tuned. In some it’s 45kw in the 4.5k range. But likely it’s 35kw from 4.5k all the way to 8k        If you add a bigger intercooler you may get more power, but what it mainly gives you is more time at those dyno numbers. Normally after slower twisting roads at high boost in lower gears the intake temps rise and it won’t make the same power as on the dyno. Or cheaper option is water spray kit.

Hi Richie,              Thanks for the info and the offer.  So I will ask for a bit of advice.  My 1999 B4 has, what we call here in America, the Subie Stumble.  It hesitates just a bit between 2,500 and 3,500 rpm.  For about half a second.   Once the stumble is done, it accelerates like normal.   Things I have done so far: 1.  New plugs and coil packs.  2.  New fuel filter.  3.  New knock sensor.  4.  New fuel line dampers.  Both of them.   5.  Cleaned idle sensor.   6.  Cleaned MAF.     Next thing is fuel pressure regulator, but that was the original question.  Any other things to check before the pressure regulator?   Thanks.    John