This depends on the application and how you drive. The determining factors are boost level, fuel octane and amount of time spent at full load on boost. The risk is detonation, which can quickly destroy cast pistons. Cast pistons are more brittle and tend to crack the ring-lands, they are also less resilient to heat spots. The more time you spend at the extremes of operation, the higher the risk you will see detonation.
If you are aiming on running higher boost, but have poor fuel octane and basic ignition control then you are at a much higher risk of detonation. Fitting forged pistons will help prevent piston damage if detonation occurs, but will not actually stop the detonation from occurring. This is where good ignition control comes into play and is an important consideration if you aim at targeting higher levels of boost. Similarly good quality fuels are essential when running at higher boost levels.
Our kits are designed to be bolted on to a stock engine and used with pump gas. In a street application the standard boost level (~6psi for the budget kit / ~8psi for the side-draft kit) is generally a safe level of boost that will not result in detonation, however we recommend that you use premium / higher octane fuel just to be safe, this increases the safety margin and reduces the risk of detonation. The main goal with the kits is for them to be a cost effective bolt-on solution and not have to change any internal components. Once you get to the stage where you are splitting the case the costs tend to escalate and what was a reasonably priced bolt on power-adder becomes a more expensive endeavour.
If you are scratch building an engine for the specific purpose of trying to maximise power by increasing capacity, adding higher flowing heads, fitting one of our cams and ignition systems along with running higher boost levels or aim to use the engine in a competition environment, then it is definitely recommend that you consider moving to forged pistons or ceramic coating a good set of cast pistons (mahle) but for a low to moderate boost street application cast pistons should be fine.
There are also some other considerations when making piston choice. Cast pistons are lighter, they are also more dimensionally stable. Forged pistons expand as they come up to operating temperature, which requires larger clearances and also means that ideally you need to wait until your engine is up to temp before loading it up to avoid premature wear. This can be a major drawback for a street motor, especially if it's a daily driver.
In most street applications that run just a moderate amount of boost (eg up to 8psi) forged pistons are not needed and you can get away with running a modded distributor.