If you're talking about G5 mod's, I'd always suggest G5 engineering at least one fresh, identical, module and then comparing it to the old (legacy) G5 module you already have.
Alternatively, I suppose you could use something like Coriolis to see how the module
should perform and then compare that to how your legacy module performs.
For example, I already had 4 AspX's, all with legacy G5 FSDs.
Luckily, I wanted a couple more (don't ask) so I bought 'em and G5'd the FSDs - under the new system.
Later on, I removed a bunch of armour and other combat junk from one of my old AspX's and realised that, without all the extra weight, it had a jump range of 79Ly!
Turned out that I must've got a "god roll" on that FSD, under the old system, but never noticed because it was fitted to a heavy ship.
Course, it
is really rare to have got a "god-roll", under the old system, but it'd be a shame if you had one and then lost it by converting the module to the new system.
In general, a G4 mod under the new system is about equal to an average G5 mod under the old system.
That being the case, when you convert a legacy G5 module to the new system - and it becomes a G4 mod - you
probably won't have lost any performance cos the new G4 will be similar to a legacy G5... unless the legacy module was a god-roll.
+EDIT+
Or, if that doesn't answer your question, a simpler way to look at it is to say that your G5 legacy modules are
probably the equivalent of a G4 mod' under the new system.
So, it's likely they
can be improved on in the new system (unless they were god-rolls) but they're not horrifically obsolete.