Lenovo Y740 - Hot after undervolt + huge temperature difference between cores

I've had my Lenovo Y740 15" for about 3 weeks now, it's the i7-9750H, GTX 1660Ti and 16GB RAM configuration. 

Performance has been great so far but I'm recently starting to notice issues with stutters in some demanding games like Battlefield 4 and Apex legends. I've used throttlestop to undervolt my CPU core and cache by -147.5 mV.

it gets better framerates than not undervolting. However, under games that are strenuous like BF4 I get really high spikes and very frequently as well, plus I'm noticing my thermals are worse than most other people's. Multiplayer and campaign gets me around 84-88C, with spikes to 92 and back. However what's weird is that the spikes are very frequent and fast. In the same area, I'll be sitting at 82 when spike to 89 then 78 then back up. This induces some slowdowns which are really annoying. 

I'm running BIOS 1.06 (latest for my configuration), with performance mode enabled and plugged in using conservative mode, with the lid shut and connected to an external monitor. Ambient temperature is around 20C where I am sitting.

I decided to run a stress test using AIDA64, and I get very different temperatures across cores. Here's the result from an FPU only stress test: 

https://imgur.com/a/Qim3tAl

As you can see some cores have a difference of almost 20C and that kind of indicates the thermal paste is dried up or improperly applied. 

I've also uploaded a picture with the results of a stress test with everything pushed, and you can see I'm hitting the 99C limiter on a few cores. What this indicates to me is a bad paste job and I want to try a repaste but if warranty is an option I would prefer that first. If I were to go through with an exchange, would I simply be able to swap the drives over when they send a replacement? I don't want to clone my drives and reinstall everything all over again as it was a massive pain in the ass the first time. I do use the laptop for work too so I would prefer if I can have no downtime. 

Let me hear your thoughts on how to proceed and if you think this is abnormal. I think it is as everyone praises the Y740 and it's possible I just got unlucky.