When mounting a probe in the downpipe,(or any pipe for that matter) I prefer the clamp in style. This simply requires drilling a 1/4″-9/32″ hole and clamping the probe in. This eliminates any rusting from welds etc. The “post turbo” installation will yield slower response and generally lower temps as some of the heat energy of the exhaust is used to drive the turbo.
Typically, the post turbo install will yield approximately 200° lower temps than “pre turbo” but will also remain higher during cool down giving good reference for turbo temp and judging appropriate shutdown time. It is generally thought that 1050° the maximum acceptable sustained temperature for operation.