Are you asking if installing the Test Server client messes up your XP in the normal game client? No, it doesn't. Totally separate program and server. XP is handled on the server side.
Here's the thing about shared XP... Unless you are doing impressive amounts of damage dealing each game to ships of your size or larger, the bulk of your XP comes from the XP your team is sharing. And THAT depends on the value of the targets they are killing.
People are seeing a dip in XP right now. Why? Because there's an event going on which is encouraging new accounts with low level ships and low level accounts.
- The same thing happened to XP when they introduced the Soviet Navy. XP dropped (through the floor in some cases) because the value of the ships being shot at and the value of the crew on board those ships dropped dramatically.
- The same thing happened back when they opened the new Nelson server (before the recent merge) and it filled with new players. Low level ships and low level crew are worth less, so XP on the server was lower than it was on the other older servers with higher level opponents.