How important is aero on a tin top?

So there is tons of books and info out there on aero these days, I've read a fair bit of it and have a general good understanding of how and why certain things work. What I do wonder is how important is this stuff on a sub 300bhp 1250kg or so race car? 

I've seen cars in this category with tons of aero not do any better in races with ones that have none. But I've also seen prestigious club race teams boasting about being  2/3 seconds faster from their carefully designed aero, but these are 50k plus built cars and products they are trying to sell too.  I've seen cars winning races and championships with all the oem flat floor, wheel liners removed which surely increases drags by a good amount. You also then see cars in between running a splitter and a wing but left the floor in a very draggy state. 

It does seem like all the aero advice is aimed at fuel economy on road cars and huge downforce on GT, formula and time attack cars but not much on the benefit to heavy lower powered saloon race cars. 

I've seen so much inconsistency that sometimes I wonder that my theories of trying out diffusers and splitters and wings will end up being a waste of time and money that could have been better spent elsewhere. 

Does anyone here have any positive or negative experience with this particular kind of car and aero that's worth sharing? 

At the 300bhp/1250kg level, you are into a range that aero effects can get a little muddy. You have enough power that drag is only a major issue at or close to top speed. At 1250kg the downforce has to be pretty significant to show up in driver feel. Both drag and downforce will affect laptimes, but again they are counter-productive to each other so the actual TOTAL laptime may not show much difference. However, segment times will show the effects: high speed straights will be somewhat slower due to drag, medium speed cornering sections will show faster due to downforce. Approaching the aero build with this in mind, you can tailor the build to suit the tracks depending on the percentage of time in corners vs straights.

Lower power cars will see more drag impact on top speed/acceleration. Heavier cars (usually more power) will need significantly more downforce to affect real benefits. The worst of both worlds is low power/heavy cars. The opposite end of the spectrum is high power/low weight cars like F1 or LMP cars, where even small amounts of downforce can bring huge benfits with little impact from drag increases.

All of the above can be summed up with: aero requires trade-offs and compromises.

GridLife's GLTC class is around 260bhp ( ~210whp) and generally around 1250kg "all up race weight".

Their rules allow 3" splitters that go back to the front wheel centerline; and rear foils that are up to 700 in^2 of area, with a maximum of 5inches of overhang from the rear of the car.  Napkin math means this is around 350#s of downforce at 80mph (i.e. ~12% of bodyweight).

On a 100sec pole lap, most of the cars without aero are usually in the 103-105sec range; and that's with a class that's nominally balanced around attempting to BoP Aero cars against non-Aero cars.  There's basically no cars there without aero, in 2023.

one of the main benefits underexplored in stafford's comments is the ability to push breaking zones with the added aero; and the confidence that that bit of extra tire grip gives in medium speed corner to push extra speed, compared to a car with purely mechanical grip.  but, yes, he's 100% right that you have to be intentional in not loading up a bunch of downforce the expense of drag when doing so.