To elaborate on what Ramrod said:
Forged pistons are more durable than cast pistons. They are better able to withstand high boost pressures and detonation.
A forged piston of identical specifications to the original piston will not increase or decrease performance. It will allow the engine to make more power reliably if there are other modifications.
There is a fairly high probability that they have a lower compression ratio than standard, but you would need to confirm this with the seller.
Lower compression ratio means that the engine will typically be a bit slower before turbo pressure builds, but it also allows the boost pressure to be higher than standard while maintaining reliability.
Final points on the forged pistons are:
they are usually assembled with slightly more clearance than cast pistons. This is NOT a problem, even if they rattle a bit when the engine is cold.
The forged piston should also take longer to wear out than a cast piston, although the difference is pretty trivial in engines designed in the last 40 years or so.
The rods are a similar story. They do nothing noticeable by themselves, but they ensure more reliability. This is particularly noticeable if the engine is running higher-than-standard boost pressure and/or being revved harder than Volvo intended.
Short version is: both of those modifications are good things even for a sedately driven car, you just need to confirm that they have not lowered the compression ratio too much.
