21 PSI Peak / 19 PSI Continuous is known as safe limit. MBC (Manual boost control) is a good option, even if using stock boost value. It changes characteristic of boost. Normally wastegate opens gradually when boost increase, when MBC fitted W/g opens when limit value is reached, so you got a nice boost response/lower lag.
An boost gauge in my L-Series:
Boost Gauge - YouTube
And new set for RONbox:
Few tips from experience:
- When choosing MBC make sure it's a "ball type" - i use Turbosmart one.
- It's very possible that your old boost pipes can't handle boost increasing , so be prepared
- I made solid boost pipes with silicone couplers - it's also make better boost response.
-Boost when adjusting should be checked with some autologic/hawkeye software. Why? because a gauge is only information for you, readings can be distorted by gauge quality, mounting hardware,measure point ect. So you should know what value of boost is "seen" by ECM
-Higher oil temps can be noticed. You should observe it. I need change oil grade to 10W60 (we've got high ambient temp on summer, and some more heat from engine/turbo) because stock 10W40 can't handle . Previous peak temp was 100-105*C on normal (City), and 110-115*C when high speed cruise/Off Road . Now it have 110*C on normal and can go up to 119-120 *C when off road/fast cruise.
p.S
Do not "fun" with boost/fueling without Boost/TIT (Turbine inlet) monitoring
it will be pretty bad when limits (650*C /1200 *F for TIT) exceeded.