Rant(ish?).
It'll cost any way you want to do it. Valve job isn't the biggest expense but I'd say it's not wasted when going for bigger power. And a mild, even correctly homemade, porting is good too. Though you can always add more boost to achieve the goal?
For a 500hp 5-cyl I would use B5234T(81mm bore vs B5254 weaker 83mm bore). H-beams and custom exhaust manifold, everything else stock. Turbo pistons are fine for about 117hp/piece. Though I'm not 100% sure about how older or newer FWD intake flow. but I'd guess they'll be fine for that. I'm not that much into the details of the intake because 6-cyl is soo much easier to drop in.
I'd guess that B5254T is good for max 350-400hp with H-beams depending on the turbo size. The bigger turbo, the less torque on low rpm to brake cylinders if that's going to happen. Most if not all higher hp engines are built with 81mm bore because they are considered stronger. B I have no idea how well 83mm bore cylinders last with having grooves between cylinders filled(about that near the end).
As for 6-cyl, I find all round driveablity way better than 5-cyl and it's easier to make the same power.
B6284T needs just about rods, change of throttlebody, RWD oil pan & engine mounts and drilling & tapping one hole for left engine mount. After that it's ready to run and has exhaust VVT. Intake flows up to about 700hp. At least I couldn't get any more than that with a T6 intake.
If you want to fiddle around, use B6254 block, stock RWD intake, H-beams, B4194T(T4), B5234T or B6284T pistons, B6304 crankshaft and B6245 or B6304 head. 204hp B6304 cams have more lift and power comes a bit easier. B6254 cams can be used too and they have same lift as B6284T. Block is good for around 600hp or even more and RWD intake works at least up to 800hp.
For all the engines above. Fill the grooves between cylinders for example with feeler gauge slices or other material which is suitable thickness. This applies to any whiteblock until the latest ones(?). Without those cylinders could and will crack under higher pressure and temperature. Block casting is thinner, doesn't support cylinders that well and especially in overheated engines casting collapses in to the groove on its own. This can be seen even in stock engines and ultimately as cracked cylinders.
ps. B6284T block itself is good for ~1000hp just with grooves filled. Friend had his "B6284T" tortured in an engine dyno for a while. Just over 1000hp and ~1150Nm with no problems. I've had only 800hp so far on mine
And due to trying to avoid problems with the engine my current block has a block reinforcement similar in the pic above. It also fills the areas between cylinders.