If you look at the DCS video by Matt Wagner about take off and landing, during the “spitfire landing” approach he says the same thing: +2 halfway in the upward turn and the spit’s speed goes down to 160, then wheels down. Watch the video closely, the boost gauge indicates -2 !
(He later does some adjustments to about 0 boost, then back down to -2).
I think its a typo from the start. And weirdly enough Matt rools back to -2 all the while saying +2...
Odd, but I use -2 and it works fine.