When using a virtual disk, I understand that on a HDD there is often a write speed penalty while the drive is still growing. But for an SSD is this penalty mostly eliminated? Is there any other advantage of using a fixed size?
I don't think that a fixed sized virtual disk give that much difference. The opposite may be true because I've experienced a lot of problem with sparse file allocation on some embedded devices. Hence using a SSD instead of a HDD doesn't simplified solving the task. A fixed disk size would just eat up all free disk space and SSD is still an expensive storage hence I wouldn't recommend to use a fixed size disk on a SSD.
It depends on what you call noticeable.
Probably the most noticable difference I see between fixed sized disks and dynamic sized disks are when you build a differencing disks. The performance gains using a differencing disk is huge if the parent disk is a fixed disk.
On an SSD, technically the penalty still exists. Just not as noticeable.
Like anything performance related, it is best to build a real example and run a benchmark. Talking about a performance situation doesn't generate performance metrics.