I have a used spec r r1 for sale.
But if you are doing well with the tub chassis why mess with a good working car?
However, with a carbon chassis car you can get closer to the min weight with better balance.
Also, is your tub chassis tweak free? I doubt it.
Setting accurate height and droop on a carbon chassis car can only help.
I raced a TA05v2 until recently. The chassis was tweaked like all ta05 tubs. And it was overweight. But I did OK.
If you buy a $400+ TC, you will immediately notice the difference in quality and handling.
But if you are on a tight budget, spec r s1 will still be a level above your TA04.
Link to my R1 in for sale section.
But let me warn you. Spec R cars are not durable like $400+ TC. The hinge pins break and the plastic pieces break in a crash more easily than XRAY, or Wildfire.