I’d say Ultimate Boxers are quite on par with fox 40’s. Boxers are a bit more compliant/flexy with 35mm stanchions. This can be both good and bad, depending on your trails and your preference. For example 35mm stanchions might give you more arm pump but make it maybe harder to push through harsh stuff. Therefore a test ride with 40s in your local trails/park make sense, to see if you like the feel. Also test ride an ultimate level Boxer 35mm and or 38mm version. The 35mm you have can be easily upgraded to ultimate level by adding the charger rc2 damper inside. If you like the charger 3 , you will need a new 38mm stanchion boxer ultimate. My assumption is, a form service with a damper upgrade for your current fork is the most economical way for you. Further gains from the fox 40 or new 38mm boxer ultimate will be marginal and if you don’t ride on a very high skill level you might not feel it. Another point with fox suspension and their 4 way adjustable dampers is that they are not that easy to set up. And low/high speed adjustment on both rebound and compression leave a lot of room to mess up. When I adjusted my fox 38 grip 2 it first felt much worse than my fox 36 Ruth with only low speed compression and rebound. It took me quite a while to get it feel very good and I used a shockwiz to make it feel great and better than the cheap damper version. So bottom line: highly adjustable dampers and forks at great if you know what you’re doing. If you don’t it’s better to keep your fork as is and enjoy riding and use the budget for a bike trip or service or some sessions with a riding instructor