could all kinds of things, when car is stone cold will it rev correctly if it does then coolant temp sensor or 02 sensor faulty,, if dont then bottom crank pulley broken and slipping loosing boost very common for rubber part to let go with age, might have a air leak on boost inlet pipework or a bypass valve issue, could also be fuel pump in tank early cars were 4.2bar later cars 5.0bar, might find timing chain is worn out rarely on r53, eed to live data run it with diagnostics tool a cheap one that works really well with them is autocom cdp plus off ebay link below for a idea

Bluetooth TCS CDP+ Plus Autocom Car & Truck Auto OBD2 OBDII Diagnostic Tool | eBay
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www.ebay.co.uk