If still within warranty, then a tuning box such as the DTUK is your best bang for your buck. You can simply unplug it when it's
servicing time.
I bought mines 2 years ago for £350, and it's the best decision I made. Once connected, which btw is very easy to do, you can control all the relevant maps and settings by Bluetooth.
Although DTUK claim the output to be 240 BHP and 500 NM, it's more like 210-220 BHP with around 420-450 NM.
However, the difference in shove compared to a stock 184 tdi, is amazing. Every now and then, I turn the box off via Bluetooth for a few days and then back on again, and you can feel the difference instantly. Leaves GTDs for dead and capable of beating GTIs.
However, if you are outwith the warranty period then a custom remap all the way. Plenty of good mappers out there who will safely extract the most out of your engine etc..
The going rate for a decent custom remap is around £400-£500. Also, you will get a dyno print with a before and after results.
Most important advice...Make sure you declare it to your insurer.