As you probably read the RS Turbo's ran warm a lot. I think it will be usefull for you to change your oil. A lot of people ran thicker oils already without problems. As you are running in an envoirment that is about 15 degrees warmer then ours you will be fine with it too and running better oil pressure.

For the boostleak the idea is that you put your complete system under pressure. For example block your exhaust after turbo and for example on the place of the meteringunit and put everything up to boost level (0.7 bar) and you will find all the leaks you have.

The dropping off boost might also be higher inlet temperatures. Everything heats up and with the higher inlet temperature ignition will start to retard. Causing you to lose boost.

You can always try the wastegate first as its easy to do. Make sure that you have the boost gauge of your friend with installing the new wastegate as you have to monitor the pressure.

If the above is not working it would suggest to buy a wideband lambda sensor and install it. Boost drop might be cause due to wrong fuelling. You have to know this else your engine might be scap soon! You can do a pre check to see if your plugs are white, then you are running lean!