I apply the Maxwell tag "hide from GI" to the sphere
This is a mistake, it does more than just let the sun through, it also stops the lightsource from contributing to second bounces (in recesses and corners) best to avoid such hacks, and also bear in mind that if you hide to gi you also need to hide from ref, if you want a correct render.
Dario has correctly recommended the best technique, he omitted to mention that you need to choose an appropriate hdr and align it, prob because thats quite obvious and didnt need saying. The other technique which is quicker but less flexible is just painting/boosting the sun spot in PS, as has been mentioned.