I’ve got a Sirion Rally 2 which has a K3-VE2 engine with the cams you mention. I’ve also got a stainless steel exhaust and have fitted (but not currently) a performance air filter/cold air intake. So I can speak with a bit of authority about those modofications!
First of all, the cams. There is a schematic on the technical section with the cam profiles of the ‘rally’ cam which you could take to an engineering shop locally (google it) and have either an existing cam re-ground (££) or have them fabricate a cam from billet (££££) to the spec. For reference, the intake cam is identical to standard, the modified cam is the exhaust cam so you actually only need one doing. The K3-VE is lower compression pistons than the K3-VE2, so you’d likely get about 90-95bhp from this modification (vs ~110bhp with the hi-compression K3-VE2). Now, the thing is that doing this moves the power further up the rev range, so yes you will get more power, but it makes the engine ‘cammy’ so you need to use the revs to get the power. And off cam, lower in the rev range, you lose power. You end up using the gearbox a lot to keep in the power. It comes on really aggressively at about 4000rpm with a K3-VE2 and there is a seam of power all the way up to 7000rpm, but below 2000rpm you’ll not pull the skin off a rice pudding. I’m sure you’ve made the connection, but using 4000rpm - 7000rpm regularly isn’t going make you popular with your neighbours…
Next up, exhaust. You can have a stainless steel exhaust fabricated for your car with a lifetime guarantee for about £600. I did, because I had a lot of trouble getting hold of an original/pattern exhaust when mine had rusted in the centre section. I recommend you go no bigger than 2.25" bore on the pipe, stock is 1.75". You will liberate a bit of power doing this, but it is NOISY. I took my car back twice because it was obnoxiously loud and in the end I modified it myself with a much bigger silencer and its still very noisy. Especially if you’re using all the revs. Which you will if you have the cams installed because off cam its slower than standard. I would hesitantly say you might see 3-5bhp from a free flowing stainless steel exhaust. My current system, which you can see on my project thread has one big silencer, I would strongly urge you to get one big and one small silencer if you’re going to do this, but space is at a premium under your copen - I was/am fortunate that the floorpan of the Sirion is built to accommodate the 4WD variant (which mine isn’t) so I have quite a bit of space to play with under there.
Next up, induction. A cone filter is cheap and easy to fit with a silicone 90 degree elbow off the throttle body and some straight pipe. You will need to drill the pipe and fit a bung and an intake temperature sensor or the EML will come on and the car will run badly. It’ll probably add 2-3bhp maybe? Oh, and make it noisier. Especially with exhaust and cam! If you are interested, I have a ready-built cold air intake with sensor boxed up that I’m not going to use which I’d sell. I’m UK based too so postage won’t be a killer and its (IIRC) plug and play.
I’m a bit of a broken record when it comes to this question, but if I were you I’d leave the engine side of things alone and focus your money on suspension and handling. Not only is it loads cheaper and easier to do, it will be a much bigger reward when driving. A set of Ultra racing chassis braces, polybushes and some decent tyres (nankang NS-2Rs for example) will be unlikely to break £600 and will give you a car that will out-handle a lot of other cars on a twisty B-road. Remember you have the best commodity on your side - lightness. Make use of it! If you have deeper pockets, a set of adjustable coilovers could improve the handling significantly too, but it’s not cheap to get a decent set of bilsteins, and I would argue that you need someone who knows what they’re doing (and how and where you will be driving) to set them up to allow you to get the most out of them. I’m going to stand aside and let @Mr_Gormsby have the stage to explain why you should soften your front springs and stiffen your rear springs (again, both cheap, easy options) to significantly improve handling. And he’s right!
TL;DR - don’t bother modifying the engine, modify the suspension and buy new tyres.
