How high can you go in a normally aspirated

[Rickskymaster]

Over this past weekend May26th, I went from Washington DC to pick-up my plane in DuPage.
A friend flew me out in his turbo Seneca.
I won't go into all of the details, but it turns out the Skymaster is still in DuPage and another trip will be necessary to pick her up. I had the new GTX 330 transponder installed.
That got me thinking on the way back. To top all of the clouds, we went up to 15,000 in the Seneca, no big deal in it. But with the
Skymaster, even though my book says Service ceiling is 18,000, what is the practical ceiling I can expect.

In other words, can I get up to 15,000 loaded in a reasonable time period.

I would really like to hear from normally aspirated guys only.

Thanks

Rick