fp8 casting is fast on GPUs that support fp8 compute.

comfy/model_management.py

@@ -647,6 +647,9 @@ def unet_dtype(device=None, model_params=0, supported_dtypes=[torch.float16, tor
         pass
 
     if fp8_dtype is not None:
+        if supports_fp8_compute(device): #if fp8 compute is supported the casting is most likely not expensive
+            return fp8_dtype
+
         free_model_memory = maximum_vram_for_weights(device)
         if model_params * 2 > free_model_memory:
             return fp8_dtype