(For road cars) it's never about the performance gain as it is personalising the car.
Top Gear did a great special on this and proved hotting up a road car was pointless, money wasting process.
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). It's a 100% true that just slapping on an aftermarket suspension and intake/exhaust mods is not going to do much of anything (other than make the car noisy and less comfortable). Likewise, swapping out brake rotors for fancy looking ones or going to a larger wheel is likely to hurt performance. But the following can make an enormous difference:
Tires: Going from a mediocre all-season to a 200 treadwear performance tire like the Bridgestone RE71R in as wide as will fit under the car is transformative. If you live in the South, temps aren't a problem. However, I will admit that I personally only use these for autocross/track day and swap wheels for daily driving.
Brakes: Most road cars need a high temp fluid and brake pad swap if you are going to play on the track from time to time.
Suspension: OEM suspensions are indeed quite good, and swapping out springs and shocks is generally unnecessary until you are getting into last ounce performance. Where suspension mods can make a HUGE difference is adding full alignment capabilities through things like adjustable control arms and camber plates- what is needed to get a true performance alignment will depend on the vehicle. Most OEM suspensions are designed to understeer and maximize tire life under daily driving conditions- this is safer for the average driver. To get the car more neutral or even slightly oversteer biased (useful if you autocross), you can stiffen up the rear with a bigger swaybar. More negative camber than the factory provides is usually necessary if you don't want to cook the outside if your tires in performance driving conditions. If you are looking for every last ounce of performance, a high-end coilover setup (not your cut-rate "stance" bro ebay special) from companies like Ohlin will improve damping and allow better corner weighting than allowed from the factory.
Power: OEM tunes have to take into account long-term reliability and emissions controls. If you throw those things out the window (or at least de-emphasize), there's no doubt you can get a lot more power- especially when you are talking about turbocharged motors where boost parameters can be changed. Throttle response can also be sharpened quite a bit, as most OEMs often intentionally introduce subtle delays for emissions purposes. Tunes for the Giulia seem to still be in the relatively early stages, but on most similar motors, 50-100hp is very possible for a tuned car with any intake an exhaust restrictions removed (OEM downpipes are usually they key restrictions). Larger turbochargers often allow 100-200 additional HP, but often require stronger internals and move the powerband up the rev range and introduce additional turbo lag.
Of course, most of these are pointless if you never autocross or track the car (and if you autocross, you have to take classing into consideration). However, you can modify a car to perform better on the track without significantly compromising the street manners for a dual-duty car. My personal daily driver (a Subaru STI) was transformed by the following modifications: downsized and lighter wheels (took away 40lbs unsprung weight), performance alignment (max negative camber, zero toe), wider 200TW summer tires, larger rear swaybar. Just these items reduced autocross run times by 2-3 seconds over the bone stock vehicle (around a 5% improvement). That doesn't sound like much, but the feel is also so much different- turn in is sharper, less trail braking required, and I no longer feel like I am fighting the car at the limit. Could Lewis Hamilton hand me a beat down in a much slower car? No doubt, but I do enjoy the car more after making the changes and it has allowed me to be competitive in local autocross competitions where before it was hopeless.