Biggest concern with Porter is his lack of spring. The guy is a bean pole and has just pedestrian vert numbers. 27" standing vert with nothing on his frame is the definition of average. Given he's not very athletic now, I doubt his body could carry around 20 more pounds without him being a plodder.
If you take him, you gotta be confident he can play at the NBA level within 10 lbs of his current weight. Given he came in at 7% body fat, he doesn't seem like a candidate to bulk up.
We've seen guys like Howard and Wade put on significant weight, yet actually increase their athleticism. Howard entered the league at 240 lbs with a vertical of 35.5". On Sport Science, he weighed in at 280 lbs, and he recorded a vertical of close to 40". It's perfectly possible to increase body weight and athleticism. It just has to be done in the right way. Durant's put on at least 20 lbs since he came into the league (235+ lbs compared to 215 lbs when he entered the league), and videos have shown that his vertical has also increased a couple of inches. Strength to weight ratios are favorable with most athletes as they put on weight. I, myself, am more athletic at 250 lbs (not to mention literally about 4x stronger) than I was at 180 lbs.
There's a reason why the world's most athletic players are also usually extremely muscular. See sprinters, for example. As they increased in body weight, their speed actually increased as well because their strength weight up at a much higher rate than their weight, thus justifying it. It all comes down to justifying the weight gains. Hell, even many of the world's most elite powerlifters and olympic weight lifters have recorded verticals in the realm of 40", and some of those guys actually have a faster 10-meter time than olympic sprinters simply due to their power. There's more to it than just "gaining weight reduces athleticism." Gaining weight and losing athleticism certainly happens, but that certainly also does not have to be the case.
Also, 7% body fat for a basketball player isn't terrible for an NBA player. 5% may be a little more preferable, but you really wouldn't want to go much lower than that. Hitting the 3% minimum required body fat for survival isn't worth the cost of acquiring it unless the person reaches that level with relative ease. Some players just naturally sit at that level of body fat without sacrificing too much, but, for other players, it isn't worth it to drop to that level or even 5%. Also, 7% body fat does not in any way correlate with Porter not being "a candidate to bulk up." That's just bro science talking there.
Suffice to say that, in my opinion, Porter could bulk up to 230 with no detriment to his athleticism (actually, while even helping it) so long as it is done correctly.