Symphony.

Anyone having experience with a build processor like mixture or prepos with Symphony? We want to opimise our webdevelopment process in 2015 ;)

I've used Mixture before a lot in the past and it's really quite good for a lot of use cases. It's fun to use and makes for a good static site generator with complex Models and Collections (something you'll be familar with if you've used backbone.js for instance and a concept similar to Sections in Symphony) for a powerful feature set. Liquid is a solid template language too which you'd likely be familar with if you've come from a Ruby/Rails background or built front-ends for Shopify.

I mainly starting moving away from Mixture as needs become more specific for the projects I was working on and teamwork required more shareable solutions so now my workflow is mostly a custom scaffold with generators built around Node.js.

I never specifically integrated Mixture with Symphony development. It is possible to run Mixture with non-mixture projects using another server such as MAMP for instance, in Simple Mode. Although you can still compile your front-end assets in that way, don't forget that XSLT is compiled on the server-side and delivered to the client so you wouldn't be able to use it in tandem with Liquid - only really for asset compilation. I'm not really sure on the overall benefit of pairing it with Symphony unless you just want to pre-build your front-end templates for integration into Symphony. Other than that, as it packs more power than a simple pre-processor it's more of an alternative to Symphony for smaller websites with no CMS requirement or static site generation.

I've not used pre-pros but I have come across it before. It's more comparable to Codekit in that it's pre-processor and asset compiler but doesn't provide the extras that Mixture does in becoming a static site generator powered by local and external models (data sources). If that's the route you want to go down then you may also want to check out Cactus. There's not really much difference between them other than Cactus uses the Django (Python) template language instead of Liquid (Ruby). The syntax is more or less identical however.

If your team is large, cross platform, or you have more specific needs then you may be better to go a custom route from the get-go. All down to scale really but Mixture is a blast. :)

Using CODA 2.5 + plugins and imageOptim for losless image compressing and Smaller for html, css and js compressor and combiner.

html, css and js compressor and combiner

HTML is compressed (sort of) by our XSLT layer when using <xsl:output indent="no" method="html"/> in your templates.

css and js compressor and combiner

There's an app extension for that... ;)

I developed this after using CodeKit, Grunt, Gulp and shell scripts for some time. Much better fit for my own workflow.

imageOptim

Should be the image extension's responsibility, but I don't think it currently has much to offer regarding optimization.