This is quite a question let's break it into a few parts :
1. first of all are you talking about textures or appearance? I will assume you are talking about appearances , visualize has the option to group by appearance which makes it easier to apply new appearances to multiple parts at once. this should not affect performance as the tessellation of the assembly is the same as if it was imported using parts grouping.
if you do find a performance drop then I would recommend using the Preview mode or use Fast Mode with the SPEED setting.
2. there are hot keys to copy an appearance from one part to another :
Holding down SHIFT and Left Clicking on the appearance you want to copy will copy it and holding down SHIFT and Right Clicking on the part you want to apply the appearance to will paste it on that part. You can use the F12 key to quickly see a list of all available hot keys.
3. you can also extract or split a part from an existing appearance group. Make sure you are in part selection mode , Right Click and select EDIT and then EXTRACT PART and LMB on the part you would like to split form the appearance group.
Note** If using Visualize 2017 you should just have to Right Click and Select SPLIT PART
Thanks for this Scott ! I love this Forum. I think I learn something new every day. The split part feature was unknown to me and will be a great time saver.
I think that this issue is important enough to periodically remind the Visualize code jockeys how this slows down a workflow. Having to essentially re-do what engineers spent considerable time doing in solidworks, is a big waste of time. Sometimes, the engineer will want to perfect his model before sending it to the graphics guys for simple "adjustment". Having to "dis-assemble" a parts-intensive model for the sole purpose of applying a (similar) appearance to different surfaces of a single part is hugely inefficient (aka PITA).
Regarding the way imported appearances are grouped...and can subsequently be modified in Visualize, sounds to me like a misunderstanding of the geometry import options... Unless I'm missing something about how the original question was described. If you import using Automatic geometry grouping, or Group/Appearance, then the breakdown of the scene tree (ie Visualize parts/groups/models) will certainly be larger. But it usually doesn't approach a scene tree size that results performance issues. (Performance problems generally creep in when using "retain structure" which creates a visualize part for every single FACE!!!) Meanwhile, importing with Automatic or Group/Appearance will allow you to more quickly change unique instances of a given appearance.
Rob Rodríguez and I did a presentation at SWW this year to help guide PV360 users in Visualize. We covered these import options in some detail... find the associated section of our PPT regarding this topic in attached PDF.
Note in particular how the scene tree differs with each import mode. This affects how appearances are applied/modified in Visualize. When using Autoamtic or Group/Appearance, you can apply new or unique appearance to a part instance that has a single appearance shared with other parts (without having to use split part!). If you want to modify the appearance attached to all the parts in one go, then edit the existing appearance properties (or use aforementioned copy/paste techniques).