There's a lot of $ put into those modifications. A properly modified G50 fully freshened with a quality LSD is big money. Same goes for a well built 3.4. But as Sal said, for that money that 3.4 better be well tuned or else it doesn't amount to much of anything.
The proof in the pudding here is that there is a ceiling as to what a car is worth, obviously. In this case where we're talking about a car with a rather high base value. So it limits how much higher the car can go IMO. Just another example of how you have to reconcile whether you're willing to pay extra to take advantage of someone else's work or else do it yourself on another car that's not as nice of an example. Either way you're going to pay more because its what you aspire the car to be. You can't have it both ways. Either you're willing to pay up for the mods or you're not. Because you know somebody else will come along who is willing to pay for them..........
You also know from your experience that driving the older cars also requires humility. Because while it may take more talent to wrangle one of these somewhat evil-handling cars, it still doesn't change the fact that the guy in the newer car is faster. Nobody forced you to buy the older car and if that's your excuse for being slower, well, you're still slower.
This is especially humbling when you're in an open class where you're governed by an open ruleset and weight : power ratio. Old vs. new with similar wt : pwr and equally talented drivers is usually going to be the newer guy coming out ahead. Translation = NASA GTS