That is certainly an interesting and affordable alteration if its permanence is not an issue.
The first question I would have would be about both how and how well the cap would actually fit your specific tub since I cannot imagine a company making individual caps for each and every tub shape ever sold. I would want the cap to actually cover at least an inch or so of the tub wall all around the cap rather than for it to merely fit inside the cutout. The issues there would be water-tightness and overall appearance after installation.
Second, and related, I would want to know how solid and stable that cap actually is as far as holding the full body weight of someone standing directly on it with both feet for a moment or two while on his or her way either into or out of the tub. Keeping in mind that the tub floor is usually higher than the bathroom floor, a large notch in the tub wall would make step-through possible in place of step-over, yet the effort and agility required to make the actual rise and step/stride from bathroom floor to tub floor do not change if one cannot step from floor to cap before then stepping on down into the tub.
If the answers to those questions were satisfactory, I would finally and again consider and evaluate the relationships between the heighths of the bathroom floor, the proposed cap/step surface and the actual tub floor. If the tub floor is relatively high above the bathroom floor, it might actually be possible and even better to simply add a riser/step to the bathroom floor (at the tub floor level) on the outside of the tub to effectively reduce the relative height of the tub wall and make step-over much easier without ever shortening the tub wall.