Yes. An inventory kit is just a shortcut for selling items stocked separately but sold together. They are especially useful when the same components might be sold as parts of different kits.
When you sell a kit, you are selling a kit, not a bunch of separate items. That’s just how the program works. When you sell an inventory item produced under a production order, you are selling that specific inventory item. The fact that you formerly had raw materials that went into its production is irrelevant. Your situation might make this seem strange, since the individual Legos in the finished inventory item could also be taken out and sold as themselves. But imagine a baker who takes input items (flour, yeast, eggs, milk) from inventory and uses them to produce pastries.
One thing you could do, regardless of which way you handle the assembled bundles of Legos, is include detailed descriptions of either the finished inventory item or the inventory kit. Use HTML to format the contents of the Description field and list whatever you want. See this Guide: Manager Guides.
Production orders will let you track average cost. Gross profit can be seen in the Inventory Profit Margin report. But that comes after the sale. Also note, there is currently a bug in the calculation of profit margin for inventory kits. See this bug report: Inventory Profit Margin report bug when Inventory Kits are sold - #7 by lubos.