diff --git a/python/sglang/multimodal_gen/configs/sample/sampling_params.py b/python/sglang/multimodal_gen/configs/sample/sampling_params.py
index 0e7563377..da728169f 100644
--- a/python/sglang/multimodal_gen/configs/sample/sampling_params.py
+++ b/python/sglang/multimodal_gen/configs/sample/sampling_params.py
@@ -116,8 +116,8 @@ class SamplingParams:
     fps: int = 24
 
     # Denoising parameters
-    num_inference_steps: int = 50
-    guidance_scale: float = 1.0
+    num_inference_steps: int = None
+    guidance_scale: float = None
     guidance_rescale: float = 0.0
     boundary_ratio: float | None = None
 
@@ -559,35 +559,26 @@ class SamplingParams:
     def _merge_with_user_params(self, user_params: "SamplingParams"):
         """
         Merges parameters from a user-provided SamplingParams object.
-
-        This method updates the current object with values from `user_params`,
-        but skips any fields that are explicitly defined in the current object's
-        subclass. This is to preserve model-specific optimal parameters.
-        It also skips fields that the user has not changed from the default
-        in `user_params`.
         """
         if user_params is None:
             return
 
-        # user is not allowed to modify any param defined in the SamplingParams subclass
-        subclass_defined_fields = set(type(self).__annotations__.keys())
+        predefined_fields = set(type(self).__annotations__.keys())
 
         # global switch: if True, allow overriding protected fields
         allow_override_protected = not user_params.no_override_protected_fields
-
         for field in dataclasses.fields(user_params):
             field_name = field.name
             user_value = getattr(user_params, field_name)
-            default_value = getattr(self, field_name)
+            default_class_value = getattr(SamplingParams, field_name)
 
-            # A field is considered user-modified if its value is different from
-            # the default
-            is_user_modified = user_value != default_value
+            # A field is considered user-modified if its value is different from the default
+            is_user_modified = user_value != default_class_value
+            is_protected_field = field_name in predefined_fields
             if is_user_modified and (
-                allow_override_protected or field_name not in subclass_defined_fields
+                allow_override_protected or not is_protected_field
             ):
-                if hasattr(self, field_name):
-                    setattr(self, field_name, user_value)
+                setattr(self, field_name, user_value)
         self.height_not_provided = user_params.height_not_provided
         self.width_not_provided = user_params.width_not_provided
         self.__post_init__()
diff --git a/python/sglang/multimodal_gen/test/server/perf_baselines.json b/python/sglang/multimodal_gen/test/server/perf_baselines.json
index 0757aa51f..45224de95 100644
--- a/python/sglang/multimodal_gen/test/server/perf_baselines.json
+++ b/python/sglang/multimodal_gen/test/server/perf_baselines.json
@@ -237,7 +237,7 @@
                 "LatentPreparationStage": 12.71,
                 "TimestepPreparationStage": 2.91,
                 "DenoisingStage": 26403.1,
-                "DecodingStage": 286.85
+                "DecodingStage": 489.8
             },
             "denoise_step_ms": {
                 "0": 511.3,