Thank you for your question, it is great that you are monitoring Simba so closely. There are a few different causes of the spots that you have described in Simba’s eyes. The brown spots are caused by growth/spread of pigmented melanocytes on the iris. Iris melanosis are spots that are not raised. If the spots are raised or have an irregular surface or alter the shape of the iris they are usually iris melanomas, this is a type of tumor. As with all tumors they can either be benign or malignant, the malignant type can spread to other organs.
Ultimately the way to differentiate between the types of spots so we can tell if they are likely to cause any problems for Simba is for him to be examined by a Veterinary Ophthalmologist. These specialists have special equipment and advanced training in veterinary eye care so they are able to confirm what type of spot Simba may have by using a slit-lamp biomicroscope.
If the specialist suspects that these could be melanomas they will discuss with you the next steps for diagnosis and treatment. It can only be determined if they are benign or malignant by a pathologist. The ophthalmologist will guide you through these steps if they feel Simba may have a melanomas.
Hopefully the spots are iris melanosis as you have wondered, luckily most of these spots do not progress into malignant tumors. However if more spots form or enlarge they can cause damage to the area where fluid in the eye drains and can then predispose cats to glaucoma - high intraocular pressures. If the ophthalmologist feels that this could be a problem for Simba they will suggest routine rechecks, possibly every 6 months, so they can check with the split lamp biomicroscope for changes to the spots and will also check his eye pressure.
I would suggest you schedule and appointment with a veterinary ophthalmologist so that they can examine Simba and give you a treatment plan if necessary.
I wish you and Simba the best of luck. Please let me know if you have any more questions.