I had a similar experience with VK100 and found the Mauschen more capable. Tracks was first upgrade, but I do not intend to upgrade the engine - instead saving the 50k XP for Maus. I felt fleeced upgrading engine on VK for no discernible difference in performance. Only improvement I can see is reduction in chance of fire on a hit from 20 to 12%, but I've not found that to be a problem to start with.
FWIW, currently at approx 100 games with my highest WR in any heavy tank so far @ 58%. The Mauschen seems to be responsible for driving our respective recent WR in opposite directions.
I can confidently suggest using heavy spall liner. Tanks this large and slow are arty magnets and receive a lot of big HE derp. Also, stock aim is already decent, I am running at a high 70s hit rate and vis is good without optics (almost max with recon and situational awareness is trained) so I am running rammer and vents to touch up performance.
Although this tank is often last to the front, I have only rarely felt that I missed the key moments either way. Sometimes, your fashionably late arrival is enough to tip the scales as the initial layout has been set.
Where the VK was very dependent on tier to have a strong impact, I find Mauschen to play its role in any match. Stick with it, be aggressive with the Maus gun, don't forget angling is still very important and try and protect your HP until you can create a later push the enemy has no answer for.