The wisps at the top of the cloud don't indicate that it's building rapidly. A crisp billowing appearance will indicate it's vertical potential and indicate that it hasn't turned to ice yet (glaciated), but you can't tell it's vertical momentum without actually seeing it move.
A rapidly building towering cumulus can quickly overtake you and you can't outclimb it...it can be climbing several thousand feet per minute very easily...what you think you can overfly will easily be thousands, even tens of thousands of feet above you in a few minutes, and can likewise quickly envelop you.
Flying through it to see what's inside is a bad idea.
You may or may not get big raindrops on your windscreen. If droplets are large enough to produce rain, it's already well into the Cb stage. If droplets are inside that are traveling upward vertically with the diameter of large raindrops, then you're in a lot of trouble at that point. Somewhere in that cell you're going to find a column of air, more than likely several, going down. Most likely that water has already made the journey more than once and is still growing, and part of what may be feeding that rising column of air will be latent heat release of the moisture beneath your altitude...which allowed the water to grow to that size of droplet, but also caused an increase in cloud column height and a vertical rise locally in the atmosphere.
When air begins rising, say at 1,200 fpm, it may meet other air which is also rising at say, 1,000 fpm. The column now rises at the rate of 2,200 fpm...the vertical velocity is cumulative. The rate is affected by the surrounding temperature, and will rise more rapidly through colder temperatures, more slowly in warmer surrounding temperatures.
The moisture content in the cloud can vary considerably with temperature and the airmass, but I certainly wouldn't fly through it to find out what's there. I've done that with a lot of scientific equipment hanging off the airplane on hardpoints and probes, measuring droplet size and moisture content and other things, and I've picked up an enormous amount of ice doing it (to say nothing of aircraft damage, hail, severe turbulence, lightening strikes and damage, controllability issues related to the turbulence and icing, and so forth). Best to fly around them...even if it's a towering cumulus. It may hide something inside that you may not like. Remember that every Cb starts life as a TCu, which may be turning nasty at any given time. Even just the rising TCu can be a killer.
All who fly instruments understand that instrument flight can be challenging under the best of circumstances, especially if one isn't flying instruments every day. Instrument flying is a perishable skill, and flying single pilot IFR is one of the highest workloads a pilot can have. Add turbulence to that, even mild turbulence, and the need for a much higher scan rate, and a higher interpretation rate exists. The potential for an upset or an unusual attitude is higher. Certainly punching clouds can be a fun endeavor, no doubt. But in the case of big clouds and tall columns of clouds that are packing energy (think of stored moisture as stored energy)...and clouds made up of energy (rising and falling air currents and columns)...there's more hazard than fun to be had.
I did a lot of column penetrating doing atmospheric research. However, when entering a cloud column or storm, I had the benefit of multiple experienced meteorologists on board providing counsel, other very experienced weather pilots alongside, and a team of meteorologists watching me on specially designed doppler radar with dedicated advanced software enhancements...something none of us get when flying from A to B punching clouds. In my case I also had a lot of excess thrust and performance that none of us have in a light single or twin.
I'd like to touch also on something I think often isn't considered when popping in and out of the clouds, and that's a system failure. This could be a vacum pump failure or an instrument failure. While flying instruments can be a challenge, flying partial panel instruments in turbulence and upset conditions can be much more so...it can also be deadly. I do a lot of night instrument flying lately, including quite a bit in light twins, and despite being current and experienced, I consider a partial panel situation to be a true emergency. Consider that in the context of experiencing an instrument loss inside one of these cells, apart from the inherent danger of the cell itself. Spooky stuff.