First of all, u said 250Whp. Then u said superb reliability. Ur term
SUPERB RELIABILITY contradicts alot of HP contribution. In order to achieve 250WHP, it's no longer little things u should be doing.
Ok, back to ur question.
1) Achieving 250WHP ,boost of minimum 1.2-1.5bar is the range depending on overall set up. Especially ur ECU. On high boost application, small turbine like TD04L will chock due to the excessive exhaust flow the turbine can handle. In order to be able to handle the exhaust, u need a bigger turbine + housing. Which will somehow cause noticeable lag.
1st contradiction to your engine of being quick. BTW, TD05 14b less laggy compare to TD05 16G. But outcome on the top end, 16G wins streets ahead...
2) Standard cams are good in idle of course. But for performance application, kelford 260 is acceptable for me... Optional springs to handle the valve float phenomena after 7000rpm. But good stuff comes with a price.
More about kelford, check following link:-
Kelford
RPW
If you want a cheaper solution, 1999 or earlier Satria GTI/Putra cams is also a good choice as the duration is a little longer and the lift is also higher. Overlapping problem face on low boost can be minimize with adjustable cam gears. But on high boost application, overlap is no longer a problem.
3) 3mm skimming on the disc is acceptable. But it's not necessary. When engine is well balanced, the smoothness is noticeable.
4) YES!!! using the CA18 conrod is recommended for really high boost application. It'll definitely lower ur compression ratio. Using a Ca18 conrod is like having a 5.6mm thick gasket. Else... U can get the China forged... Quite reliable actually dur to it's H beam configuration.
5) Why do you need to replace the valve when there's nothing wrong with ur current valves... If u need oversizing, try ask around spareparts shop. I've seen overgrinded valve seats and replaced with 0.5mm oversized. Makes alot of difference interms of flow.
Ignore the costy valve job, costly
GSR highcams, costly
GSR internals. My suggestion would be, make ur fuel tunning effective. Standalone + TD05 + 450cc injectors + Minimum of VR4 intercooler. If budget allows, go for stiffer valve spring since ur ECU has been upgraded to 10krpm rev cut. If u still have lotsa money, then go for an reputated external wastegate.
1 last piece of information, Stock without any modification is the most reliable. Happy modding :)