V50 Engine Turbo : Performance issue possibly ruled down to ECU?

[Xariez]

Heyya,

I have been diagnosing a performance related issue for quite some time now, with a Volvo V50 T5 2004. On and off these very forums, other forums, sleepless nights and endless days about what could be going on. As a base, the OBD2 scanner reports no weird readings, no error codes, nothing. Same goes for the car, no kind of LIMP MODE or anything has shown up.

However, here's the interesting part ..

I had the car in for a cooler hose replacement the other day that needed replacing quite badly, which also prompted the battery (and ECU!) to be disconnected for a day or two while ordering parts and swapping it out. Took it out for a spin tonight, and the performance is back! So in my head, this gives us a few possible options:

1) Performance Loss due to coolant fluid/pressure leak - I have absolutely no idea why this would be a thing, but within the right conditions it's probably a remote possibility.

2) Performance Loss due to the placebo effect - I haven't driven it for a couple of days, and as such the "surge" was more enjoyable. Again, I feel like it was so significant that it wouldn't be up to placebo, but it is certainly within the realms of possibility.

3) Performance Loss from earlier and fixed due to the battery being disconnected, and as such a ECU reset (and perhaps something else I am unaware of?) - this is, in my head, the most likely reason.

4) There's no "performance loss" going on at all, but it's rather a "performance boost" that is being gained from performing the reset, and as such all this worrying is a waste. I will admit I am very unsure about whether this is a thing or not, but as it is something I believe I've heard about, I might as well consider it.

I'm not as much interested in the reason, as much as I am why it happened, if it'll happen again, and what can be done to make sure it doesn't happen at all. I am truly at a loss and any and all help is greatly appreciated. I will also keep this post updated in case it starts slowing down again.

As for the OBD results I talked about earlier, after tonights run they seem very similar to what I had before. Around 300KPA fuel pressure, and around 0.3-0.6 bars of boost (depending on the weight of my right foot).