Colby here from Shogun Page Builder. The Shogun page building app does have the ability to make image sliders go across the full width of the page using our full-width option that is available on elements in the app. This can also be done with other sections on pages as well through our app. If you'd like to give the Shogun Page Builder a try and test this for your store, we have a 10 day free trial on all new installations!
Thank you for reaching out to the community about that. If you are using a free Shopify theme and are eligible for design time, then our themes team would be able to remove the whitespace between your header and your slideshow for you. Let me know if you'd like to me to ask them to make that change for you and I can get that started!
Making the slideshow full width would not a supported customization, so for that you could try contacting one of our Shopify Experts, or using an app as Colby mentioned above (thanks for the tip @Colby_W)! If you are using a paid theme by a third party developer, then I'd suggest reaching out to them directly to see if they can support those customizations for you. Let me know if you have any further questions about that.