I think I remember hearing 300* difference, hotter at the manifold, but never believed or trusted it. Mine is in the manifold.
Its accurate but does not reflect the temperature you want to see, manifold temperature will show the engine exhaust temps rather than including the temperature drop across the turbo. The temperature drop across the turbo reflects how much energy the turbo is using to develop boost.
I had dual Isspro EV2 pyros since banks put the sensor boss in the downpipe for ease of installation. Later installed another one in the manifold. Steady state cruising, they read fairly close, about 50* as I recall. WOT uphill with 10k trailer, about 400* higher at the manifold and climbing while indicating 1050* at downpipe with crappy banks ottomind chip. Backed out of it real quick and got new tunes. LOL