What exactly is the issue in detail?
I had some boost issues with my 2006 D40 STX. If you are driving and you find it's boosting then it cuts out suddenly, that will be your ECU sensing overboost and cutting the fuel(this happens at 21psi)
If this is the case, i'd be getting a boost gauge fitted to make sure you aren't overboosting too much which can be problematic long term.
If it's not getting enough boost (feels laggy and slow like it's struggling bigtime) as Tony said, first point of call is always the vacuum lines. Check every line from that boost solenoid you replaced. Two of them will cross over the top of the engine manifold to the suction control valve on the battery side of the block. Check for tiny splits (even the smallest crack will cause an issue).
The other problem may be the SCV (suction control valve) itself. Symptoms are generally the same as when you have a split line or similar (laggy, slow, lack of power).
Absolute worst case, your turbo seals might be shot. You will know this though if you get under the car and look around the turbo area you will see oil around the turbo. That's generally not the case though, 8/10 it's to do with vacuum / boost solenoid / SCV.
Also try disconnecting your battery for an hour or so, then reconnect as this will do a hard reset on your ECU, sometimes they go into limp mode for whatever reason. Reset will usually clear that code that has caused the limp mode.