Sounds very much like fuel cut, I assume that they logged the rail pressure?
Were there any missfires detected through diagnostics?
Sounds identical to an Issue we had recently with a leon K1, would flatten off at 4krpm and occasionally missfire, setting changes helped when we removed some boost from the map (wasnt revo, one of our own codes) and leaned out the AFR but came back in only 5th gear, we looked into the valve bounce issue but weren't convinced this was the cause so as a test we logged rail pressures with high boost settings and as we richened AFR's the rail pressure would max out and pull ignition timing. Fitted a HPFP internals kit as a test and problem went away, so much so that we could increase timing and boost midrange and picked up an extra 12hp and 21 ft/lb. Requested rail pressures were increased to approx 128 bar.
Car returned 5 weeks later for new plugs and coilpacks as we discovered a missfire on cyl3 but its been perfect since. Find a friendly forum member with vagcom thats local to you and get logging, bet the rai. Pressure is struggling
Im answering calls all weekend (drew the short straw so im working) incase you need any help.
Dave@PDT