I can confirm that altough LR 5.3 RC now does have a non-zero default setting for the feathering which makes the images that include spot healing much more look like in LR4, there is still a quite noticable difference (in fact, it appears that the transparency curve is different between both versions). The difference is noticable in particular when the diameter of the spot is close to the size of the object to be removed (i.e. there is usually no problems when removing skin blemishes etc.) .
I believe it is of paramount importance for the LR users that the images are rendered the exact same way when upgrading to a new version. This is why LR uses the concept of process versions. Note that the images may comprise a great deal of manual work iun LR to make them look as desired. For many photograhers, it is important that the look of exported images that are sent to the customer don change just because the software was upgraded.
There might be two solutions to fix the problem in the software:
1. Use the LR4 feathering if the settings of the tool has not been altered by the user and switch to the new rendering as soon as the user touches the feathering slider. This avoids introducing a process version now that the third version of LR5 is out. However, it might be a bit tricky to communicate this behaviour to the customer.
2. Introduce a new process version (and do so in future whenever the slightest change is introduced in the rendering engine).