Just throwing random tuner parts at a car is pointless, but that doesn't mean road cars can't be made quicker through tuning.
I'm not sure which segment of Top Gear you are referring to, but I seem to recall they did things like just throw a new shock and spring package and a noisemaker straw (cold air intake).
I think this statement is unfair on what they did in the segment I watched as they did make the car handle much more consistent, just not quicker over a single lap. Until they started playing with air dams and wings, they were making valid and noticeable changes. And I do believe the difference is once the car was modified it could have achieved that lap time close to all day long, but when standard you would have only been able to do that lap time once (because judging the apex of a corner when the car is rolling all over the place is not easy compared to a composed flat sitting chassis, brake fad, tyre temperatures uneven meaning the (likely) outer edge of the tyre will soon loose it's effectiveness, etc.
The point is, and yes agree with Michael, if you put Lewis Hamilton in your dead standard 105 series Alfa he would be able to do at least one very quick lap. Most likely quicker than you, but even he would not be able to lap at that pace in your car all day long for the above reasons. So the stop watch records a time of X. This is how the TopGear segment worked. They put the Stig in the standard car and said "do your best time": I imagine 30 minutes later he had found the best time, potentially with some slow laps to let things cool down in between. The Stig can drive, so yep he was going to be able to pull a single really good lap out.
You then spend all that money on "tuning" the car, and you might end up with the stop watch recording time of X - a little bit, but unless competing semi-seriously the return on investment to 99.99999999999999999999999999% of people on this planet would not be there. That was there point. Unless you need your car to be able to handle a continuous beating, it is quite simply a waste of money and an "I want to play with my car" hobby.
Example, lets use my 156v6 to demonstrate their point: I drive an hour of open road to work. Sometimes I take the 156v6. Is there ever going to be a time when I need that car to with stand being HAMMERED from home all the way to work? Absolutely never, so I will be able to drive sufficiently well in the dead standard car to get to work in hurry if I need to. If I firmed up the suspension a little, I might be able to take 3 or 4 corners a bit faster and it might feel a bit more refined, but over the whole hour that modification is me being a bit of a wanker. Might as well put a go faster sticker on the rear window.
BUT if I attended the May Madness event that the AROC NZ club puts on at the Manifeld race track every year and wanted to be able to do as many laps as possible: yep in goes the fade resistant pads, bleed the brakes, if serious sure a few suspension modifications (dial in some more negative camber), better tyres, etc. would make that event a hoot. But if I took her dead standard to the track I could get very close to the same lap time in one heroic and hard lap ... but I'd probably cause considerable wear to the tyres, etc. ... and she would get slower through out the day, unless I kept throwing new tyres at her, etc.
TopGear are/were right; when you see the young kids (usually, but sometimes a few older bearded AROC members

) with their lowered and uncomfortable cars bouncing down the road, with an engine idle affected by some tuning, unless they use that car on the race track/auto course/etc. events regularly they have wasted their money, and are a bit of a tool.
In fact car manufacturers know this and when you purchase a GT version of a car, guess what, most often the engine and suspension is EXACTLY the same as the base model, the GT simply has some stripes and more racy interior. Hilarious but of course the GT owner buys the GT because he is a car enthusiast and the stripes and trim make him/her feel better, and it will never ever see a race track.
Pete